On Linux, PPP functionality is split into two parts: a kernel component that handles the low-level protocols (HDLC, IPCP, IPXCP, etc.) and the user space pppd daemon that handles the various higher-level protocols, such as PAP and CHAP. The current release of the PPP software for Linux contains the PPP daemon pppd and a program named chat that automates the dialing of the remote system.
The PPP kernel driver was written by Michael Callahan and reworked by Paul Mackerras. pppd was derived from a free PPP implementation[3] for Sun and 386BSD machines that was written by Drew Perkins and others, and is maintained by Paul Mackerras. It was ported to Linux by Al Longyear. chat was written by Karl Fox.
If you have any general questions about PPP, ask the people on the Linux-net mailing list at vger.rutgers.edu.
Like SLIP, PPP is implemented by a special line discipline. To use a serial line as a PPP link, you first establish the connection over your modem as usual, and subsequently convert the line to PPP mode. In this mode, all incoming data is passed to the PPP driver, which checks the incoming HDLC frames for validity (each HDLC frame carries a 16-bit checksum), and unwraps and dispatches them. Currently, PPP is able to transport both the IP protocol, optionally using Van Jacobson header compression, and the IPX protocol.
pppd aids the kernel driver, performing the initialization and authentication phase that is necessary before actual network traffic can be sent across the link. pppd's behavior may be fine-tuned using a number of options. As PPP is rather complex, it is impossible to explain all of them in a single chapter. This book therefore cannot cover all aspects of pppd, but only gives you an introduction. For more information, consult Using & Managing PPP or the pppd manual pages, and READMEs in the pppd source distribution, which should help you sort out most questions this chapter fails to discuss. The PPP-HOWTO might also be of use.
Probably the greatest help you will find in configuring PPP will come from other users of the same Linux distribution. PPP configuration questions are very common, so try your local usergroup mailing list or the IRC Linux channel. If your problems persist even after reading the documentation, you could try the comp.protocols.ppp newsgroup. This is the place where you can find most of the people involved in pppd development.
Read More..
iklan
source
Jumat, 26 Desember 2008
All About ifconfig
There are many more parameters to ifconfig than we have described so far. Its normal invocation is this:
ifconfig interface [address [parameters]]
interface is the interface name, and address is the IP address to be assigned to the interface. This may be either an IP address in dotted quad notation or a name that ifconfig will look up in /etc/hosts. If ifconfig is invoked with only the interface name, it displays that interface's configuration. When invoked without any parameters, it displays all interfaces you have configured so far; a -a option forces it to show the inactive ones as well. A sample invocation for the Ethernet interface eth0 may look like this:
# ifconfig eth0
eth0 Link encap 10Mbps Ethernet HWaddr 00:00:C0:90:B3:42
inet addr 172.16.1.2 Bcast 172.16.1.255 Mask 255.255.255.0
UP BROADCAST RUNNING MTU 1500 Metric 0
RX packets 3136 errors 217 dropped 7 overrun 26
TX packets 1752 errors 25 dropped 0 overrun 0
The MTU and Metric fields show the current MTU and metric value for that interface. The metric value is traditionally used by some operating systems to compute the cost of a route. Linux doesn't use this value yet, but defines it for compatibility, nevertheless.
The RX and TX lines show how many packets have been received or transmitted error free, how many errors occurred, how many packets were dropped (probably because of low memory), and how many were lost because of an overrun. Receiver overruns usually occur when packets come in faster than the kernel can service the last interrupt. The flag values printed by ifconfig roughly correspond to the names of its command-line options; they will be explained later.
The following is a list of parameters recognized by ifconfig with the corresponding flag names. Options that simply turn on a feature also allow it to be turned off again by preceding the option name by a dash (-).
up
This option makes an interface accessible to the IP layer. This option is implied when an address is given on the command line. It may also be used to reenable an interface that has been taken down temporarily using the down option. This option corresponds to the flags UP and RUNNING.
down
This option marks an interface inaccessible to the IP layer. This effectively disables any IP traffic through the interface. Note that this option will also automatically delete all routing entries that use this interface.
netmask mask
This option assigns a subnet mask to be used by the interface. It may be given as either a 32-bit hexadecimal number preceded by 0x, or as a dotted quad of decimal numbers. While the dotted quad format is more common, the hexadecimal representation is often easier to work with. Netmasks are essentially binary, and it is easier to do binary-to-hexadecimal than binary-to-decimal conversion.
pointopoint address
This option is used for point-to-point IP links that involve only two hosts. This option is needed to configure SLIP or PLIP interfaces, for example. If a point-to-point address has been set, ifconfig displays the POINTOPOINT flag.
broadcast address
The broadcast address is usually made up from the network number by setting all bits of the host part. Some IP implementations (systems derived from BSD 4.2, for instance) use a different scheme in which all host part bits are cleared instead. The broadcast option adapts to these strange environments. If a broadcast address has been set, ifconfig displays the BROADCAST flag.
IRQ
This option allows you to set the IRQ line used by certain devices. This is especially useful for PLIP, but may also be useful for certain Ethernet cards.
metric number
This option may be used to assign a metric value to the routing table entry created for the interface. This metric is used by the Routing Information Protocol (RIP) to build routing tables for the network. The default metric used by ifconfig is zero. If you don't run a RIP daemon, you don't need this option at all; if you do, you will rarely need to change the metric value. RIP chooses the optimal route to a given host based on the "length" of the path. It is computed by summing up the individual metric values of each host-to-host link. By default, a hop has length 1, but this may be any positive integer less than 16. (A route length of 16 is equal to infinity. Such routes are considered unusable.) The metric parameter sets this hop cost, which is then broadcast by the routing daemon.
mtu bytes
This sets the Maximum Transmission Unit, which is the maximum number of octets the interface is able to handle in one transaction. For Ethernets, the MTU defaults to 1,500 (the largest allowable size of an Ethernet packet); for SLIP interfaces, it is 296. (There is no constraint on the MTU of SLIP links; this value is a good compromise.)
arp
This is an option specific to broadcast networks such as Ethernets or packet radio. It enables the use of the Address Resolution Protocol (ARP) to detect the physical addresses of hosts attached to the network. For broadcast networks, it is on by default. If ARP is disabled, ifconfig displays the NOARP flag.
-arp
This option disables the use of ARP on this interface.
promisc
This option puts the interface in promiscuous mode. On a broadcast network, this makes the interface receive all packets, regardless of whether they were destined for this host or not. This allows network traffic analysis using packet filters and such, also called Ethernet snooping. Usually, this is a good technique for hunting down network problems that are otherwise hard to detect. Tools such as tcpdump rely on this.
On the other hand, this option allows attackers to do nasty things, such as skim the traffic of your network for passwords. You can protect against this type of attack by prohibiting just anyone from plugging their computers into your Ethernet. You could also use secure authentication protocols, such as Kerberos or the
secure shell login suite.[11] This option corresponds to the PROMISC flag.
ssh can be obtained from ftp.cs.hut.fi in /pub/ssh.
-promisc
This option turns promiscuous mode off.
allmulti
Multicast addresses are like Ethernet broadcast addresses, except that instead of automatically including everybody, the only people who receive packets sent to a multicast address are those programmed to listen to it. This is useful for applications like Ethernet-based videoconferencing or network audio, to which only those interested can listen. Multicast addressing is supported by most, but not all, Ethernet drivers. When this option is enabled, the interface receives and passes multicast packets for processing.
This option corresponds to the ALLMULTI flag.
-allmulti
This option turns multicast addresses off. Read More..
ifconfig interface [address [parameters]]
interface is the interface name, and address is the IP address to be assigned to the interface. This may be either an IP address in dotted quad notation or a name that ifconfig will look up in /etc/hosts. If ifconfig is invoked with only the interface name, it displays that interface's configuration. When invoked without any parameters, it displays all interfaces you have configured so far; a -a option forces it to show the inactive ones as well. A sample invocation for the Ethernet interface eth0 may look like this:
# ifconfig eth0
eth0 Link encap 10Mbps Ethernet HWaddr 00:00:C0:90:B3:42
inet addr 172.16.1.2 Bcast 172.16.1.255 Mask 255.255.255.0
UP BROADCAST RUNNING MTU 1500 Metric 0
RX packets 3136 errors 217 dropped 7 overrun 26
TX packets 1752 errors 25 dropped 0 overrun 0
The MTU and Metric fields show the current MTU and metric value for that interface. The metric value is traditionally used by some operating systems to compute the cost of a route. Linux doesn't use this value yet, but defines it for compatibility, nevertheless.
The RX and TX lines show how many packets have been received or transmitted error free, how many errors occurred, how many packets were dropped (probably because of low memory), and how many were lost because of an overrun. Receiver overruns usually occur when packets come in faster than the kernel can service the last interrupt. The flag values printed by ifconfig roughly correspond to the names of its command-line options; they will be explained later.
The following is a list of parameters recognized by ifconfig with the corresponding flag names. Options that simply turn on a feature also allow it to be turned off again by preceding the option name by a dash (-).
up
This option makes an interface accessible to the IP layer. This option is implied when an address is given on the command line. It may also be used to reenable an interface that has been taken down temporarily using the down option. This option corresponds to the flags UP and RUNNING.
down
This option marks an interface inaccessible to the IP layer. This effectively disables any IP traffic through the interface. Note that this option will also automatically delete all routing entries that use this interface.
netmask mask
This option assigns a subnet mask to be used by the interface. It may be given as either a 32-bit hexadecimal number preceded by 0x, or as a dotted quad of decimal numbers. While the dotted quad format is more common, the hexadecimal representation is often easier to work with. Netmasks are essentially binary, and it is easier to do binary-to-hexadecimal than binary-to-decimal conversion.
pointopoint address
This option is used for point-to-point IP links that involve only two hosts. This option is needed to configure SLIP or PLIP interfaces, for example. If a point-to-point address has been set, ifconfig displays the POINTOPOINT flag.
broadcast address
The broadcast address is usually made up from the network number by setting all bits of the host part. Some IP implementations (systems derived from BSD 4.2, for instance) use a different scheme in which all host part bits are cleared instead. The broadcast option adapts to these strange environments. If a broadcast address has been set, ifconfig displays the BROADCAST flag.
IRQ
This option allows you to set the IRQ line used by certain devices. This is especially useful for PLIP, but may also be useful for certain Ethernet cards.
metric number
This option may be used to assign a metric value to the routing table entry created for the interface. This metric is used by the Routing Information Protocol (RIP) to build routing tables for the network. The default metric used by ifconfig is zero. If you don't run a RIP daemon, you don't need this option at all; if you do, you will rarely need to change the metric value. RIP chooses the optimal route to a given host based on the "length" of the path. It is computed by summing up the individual metric values of each host-to-host link. By default, a hop has length 1, but this may be any positive integer less than 16. (A route length of 16 is equal to infinity. Such routes are considered unusable.) The metric parameter sets this hop cost, which is then broadcast by the routing daemon.
mtu bytes
This sets the Maximum Transmission Unit, which is the maximum number of octets the interface is able to handle in one transaction. For Ethernets, the MTU defaults to 1,500 (the largest allowable size of an Ethernet packet); for SLIP interfaces, it is 296. (There is no constraint on the MTU of SLIP links; this value is a good compromise.)
arp
This is an option specific to broadcast networks such as Ethernets or packet radio. It enables the use of the Address Resolution Protocol (ARP) to detect the physical addresses of hosts attached to the network. For broadcast networks, it is on by default. If ARP is disabled, ifconfig displays the NOARP flag.
-arp
This option disables the use of ARP on this interface.
promisc
This option puts the interface in promiscuous mode. On a broadcast network, this makes the interface receive all packets, regardless of whether they were destined for this host or not. This allows network traffic analysis using packet filters and such, also called Ethernet snooping. Usually, this is a good technique for hunting down network problems that are otherwise hard to detect. Tools such as tcpdump rely on this.
On the other hand, this option allows attackers to do nasty things, such as skim the traffic of your network for passwords. You can protect against this type of attack by prohibiting just anyone from plugging their computers into your Ethernet. You could also use secure authentication protocols, such as Kerberos or the
secure shell login suite.[11] This option corresponds to the PROMISC flag.
ssh can be obtained from ftp.cs.hut.fi in /pub/ssh.
-promisc
This option turns promiscuous mode off.
allmulti
Multicast addresses are like Ethernet broadcast addresses, except that instead of automatically including everybody, the only people who receive packets sent to a multicast address are those programmed to listen to it. This is useful for applications like Ethernet-based videoconferencing or network audio, to which only those interested can listen. Multicast addressing is supported by most, but not all, Ethernet drivers. When this option is enabled, the interface receives and passes multicast packets for processing.
This option corresponds to the ALLMULTI flag.
-allmulti
This option turns multicast addresses off. Read More..
Interface Configuration for IP
After setting up your hardware as explained in Chapter 4, Configuring the Serial Hardware, you have to make these devices known to the kernel networking software. A couple of commands are used to configure the network interfaces and initialize the routing table. These tasks are usually performed from the network initialization script each time you boot the system. The basic tools for this process are called ifconfig (where "if" stands for interface) and route. ifconfig is used to make an interface accessible to the kernel networking layer. This involves the assignment of an IP address and other parameters, and activation of the interface, also known as "bringing up" the interface. Being active here means that the kernel will send and receive IP datagrams through the interface. The implest way to invoke it is with:
ifconfig interface ip-address
This command assigns ip-address to interface and activates it. All other parameters are set to default values. For instance, the default network mask is derived from the network class of the IP address, such as 255.255.0.0 for a class B address. ifconfig is described in detail in the section "All About ifconfig". route allows you to add or remove routes from the kernel routing table. It can be invoked as:
route [add|del] [-net|-host] target [if]
The add and del arguments determine whether to add or delete the route to target. The -net and -host arguments tell the route command whether the target is a network or a host (a host is assumed if you don't specify). The if argument is again optional, and allows you to specify to which network interface the route should be directed -- the Linux kernel makes a sensible guess if you don't supply this information. This topic will be explained in more detail in succeeding sections.
The Loopback Interface
The very first interface to be activated is the loopback interface:
# ifconfig lo 127.0.0.1
Occasionally, you will see the dummy hostname localhost being used instead of the IP address. ifconfig will look up the name in the hosts file, where an entry should declare it as the hostname for 127.0.0.1:
# Sample /etc/hosts entry for localhost
localhost 127.0.0.1
To view the configuration of an interface, you invoke ifconfig, giving it only the interface name as argument:
$ ifconfig lo
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:3924 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
Collisions:0
As you can see, the loopback interface has been assigned a netmask of 255.0.0.0, since 127.0.0.1 is a class A address.
Now you can almost start playing with your mini-network. What is still missing is an entry in the routing table that tells IP that it may use this interface as a route to destination 127.0.0.1. This is accomplished by using:
# route add 127.0.0.1
Again, you can use localhost instead of the IP address, provided you've entered it into your /etc/hosts. Next, you should check that everything works fine, for example by using ping. ping is the networking equivalent of a sonar device. The command is used to verify that a given address is actually reachable, and to measure the delay that occurs when sending a datagram to it and back again. The time required for this process is often referred to as the "round-trip time":
Anyone remember Pink Floyd's "Echoes"?
# ping localhost
PING localhost (127.0.0.1): 56 data bytes
64 bytes from 127.0.0.1: icmp_seq=0 ttl=255 time=0.4 ms
64 bytes from 127.0.0.1: icmp_seq=1 ttl=255 time=0.4 ms
64 bytes from 127.0.0.1: icmp_seq=2 ttl=255 time=0.4 ms
^C
--- localhost ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max = 0.4/0.4/0.4 ms
#
When you invoke ping as shown here, it will continue emitting packets forever, unless interrupted by the user.
The ^C marks the place where we pressed Ctrl-C.
The previous example shows that packets for 127.0.0.1 are properly delivered and a reply is returned to ping almost instantaneously. This shows that you have successfully set up your first network interface. If the output you get from ping does not resemble that shown in the previous example, you are in trouble. Check any errors if they indicate that some file hasn't been installed properly. Check that the ifconfig and route binaries you use are compatible with the kernel release you run, and above all, that the kernel has been
compiled with networking enabled (you see this from the presence of the /proc/net directory). If you get an error message saying "Network unreachable," you probably got the route command wrong. Make sure you use the same address you gave to ifconfig.
The steps previously described are enough to use networking applications on a standalone host. After adding the lines mentioned earlier to your network initialization script and making sure it will be executed at boot time, you may reboot your machine and try out various applications. For instance, telnet localhost should establish a telnet connection to your host, giving you a login: prompt.
However, the loopback interface is useful not only as an example in networking books, or as a test bed during development, but is actually used by some applications during normal operation. Therefore, you always have to configure it, regardless of whether your machine is attached to a network or not.
For example, all applications based on RPC use the loopback interface to register themselves with the portmapper daemon at startup. These applications include NIS and NFS. Read More..
ifconfig interface ip-address
This command assigns ip-address to interface and activates it. All other parameters are set to default values. For instance, the default network mask is derived from the network class of the IP address, such as 255.255.0.0 for a class B address. ifconfig is described in detail in the section "All About ifconfig". route allows you to add or remove routes from the kernel routing table. It can be invoked as:
route [add|del] [-net|-host] target [if]
The add and del arguments determine whether to add or delete the route to target. The -net and -host arguments tell the route command whether the target is a network or a host (a host is assumed if you don't specify). The if argument is again optional, and allows you to specify to which network interface the route should be directed -- the Linux kernel makes a sensible guess if you don't supply this information. This topic will be explained in more detail in succeeding sections.
The Loopback Interface
The very first interface to be activated is the loopback interface:
# ifconfig lo 127.0.0.1
Occasionally, you will see the dummy hostname localhost being used instead of the IP address. ifconfig will look up the name in the hosts file, where an entry should declare it as the hostname for 127.0.0.1:
# Sample /etc/hosts entry for localhost
localhost 127.0.0.1
To view the configuration of an interface, you invoke ifconfig, giving it only the interface name as argument:
$ ifconfig lo
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:3924 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
Collisions:0
As you can see, the loopback interface has been assigned a netmask of 255.0.0.0, since 127.0.0.1 is a class A address.
Now you can almost start playing with your mini-network. What is still missing is an entry in the routing table that tells IP that it may use this interface as a route to destination 127.0.0.1. This is accomplished by using:
# route add 127.0.0.1
Again, you can use localhost instead of the IP address, provided you've entered it into your /etc/hosts. Next, you should check that everything works fine, for example by using ping. ping is the networking equivalent of a sonar device. The command is used to verify that a given address is actually reachable, and to measure the delay that occurs when sending a datagram to it and back again. The time required for this process is often referred to as the "round-trip time":
Anyone remember Pink Floyd's "Echoes"?
# ping localhost
PING localhost (127.0.0.1): 56 data bytes
64 bytes from 127.0.0.1: icmp_seq=0 ttl=255 time=0.4 ms
64 bytes from 127.0.0.1: icmp_seq=1 ttl=255 time=0.4 ms
64 bytes from 127.0.0.1: icmp_seq=2 ttl=255 time=0.4 ms
^C
--- localhost ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max = 0.4/0.4/0.4 ms
#
When you invoke ping as shown here, it will continue emitting packets forever, unless interrupted by the user.
The ^C marks the place where we pressed Ctrl-C.
The previous example shows that packets for 127.0.0.1 are properly delivered and a reply is returned to ping almost instantaneously. This shows that you have successfully set up your first network interface. If the output you get from ping does not resemble that shown in the previous example, you are in trouble. Check any errors if they indicate that some file hasn't been installed properly. Check that the ifconfig and route binaries you use are compatible with the kernel release you run, and above all, that the kernel has been
compiled with networking enabled (you see this from the presence of the /proc/net directory). If you get an error message saying "Network unreachable," you probably got the route command wrong. Make sure you use the same address you gave to ifconfig.
The steps previously described are enough to use networking applications on a standalone host. After adding the lines mentioned earlier to your network initialization script and making sure it will be executed at boot time, you may reboot your machine and try out various applications. For instance, telnet localhost should establish a telnet connection to your host, giving you a login: prompt.
However, the loopback interface is useful not only as an example in networking books, or as a test bed during development, but is actually used by some applications during normal operation. Therefore, you always have to configure it, regardless of whether your machine is attached to a network or not.
For example, all applications based on RPC use the loopback interface to register themselves with the portmapper daemon at startup. These applications include NIS and NFS. Read More..
Writing hosts and networks Files
After you have subnetted your network, you should prepare for some simple sort of hostname resolution using the /etc/hosts file. If you are not going to use DNS or NIS for address resolution, you have to put all hosts in the hosts file. Even if you want to run DNS or NIS during normal operation, you should have some subset of all hostnames in /etc/hosts. You should have some sort of name resolution, even when no network interfaces are running, for example, during boot time. This is not only a matter of convenience, but it allows you to use symbolic hostnames in your network rc scripts. Thus, when changing IP addresses, you only have to copy an updated hosts file to all machines and reboot, rather than edit a large number of rc files separately. Usually you put all local hostnames and addresses in hosts, adding those of any gateways and NIS servers used.
You need the address of an NIS server only if you use Peter Eriksson's NYS. Other NIS implementations locate their servers only at runtime by using ypbind. You should make sure your resolver only uses information from the hosts file during initial testing. Sample files that come with your DNS or NIS software may produce strange results. To make all applications use /etc/hosts exclusively when looking up the IP address of a host, you have to edit the /etc/host.conf file. Comment out any lines that begin with the keyword order by preceding them with a hash sign, and insert the line: order hosts The configuration of the resolver library is covered in detail in Chapter 6, Name Service and Resolver Configuration.
The hosts file contains one entry per line, consisting of an IP address, a hostname, and an optional list of aliases for the hostname. The fields are separated by spaces or tabs, and the address field must begin in the first column. Anything following a hash sign (#) is regarded as a comment and is ignored. Hostnames can be either fully qualified or relative to the local domain. For vale, you would usually enter the fully qualified name, vale.vbrew.com, and vale by itself in the hosts file, so that it is known by both its official name and the shorter local name.
This is an example how a hosts file at the Virtual Brewery might look. Two special names are included, vlager-if1 and vlager-if2, which give the addresses for both interfaces used on vlager:
#
# Hosts file for Virtual Brewery/Virtual Winery
#
# IP FQDN aliases
#
127.0.0.1 localhost
#
172.16.1.1 vlager.vbrew.com vlager vlager-if1
172.16.1.2 vstout.vbrew.com vstout
172.16.1.3 vale.vbrew.com vale
#
172.16.2.1 vlager-if2
172.16.2.2 vbeaujolais.vbrew.com vbeaujolais
172.16.2.3 vbardolino.vbrew.com vbardolino
172.16.2.4 vchianti.vbrew.com vchianti
Just as with a host's IP address, you should sometimes use a symbolic name for network numbers, too. Read More..
You need the address of an NIS server only if you use Peter Eriksson's NYS. Other NIS implementations locate their servers only at runtime by using ypbind. You should make sure your resolver only uses information from the hosts file during initial testing. Sample files that come with your DNS or NIS software may produce strange results. To make all applications use /etc/hosts exclusively when looking up the IP address of a host, you have to edit the /etc/host.conf file. Comment out any lines that begin with the keyword order by preceding them with a hash sign, and insert the line: order hosts The configuration of the resolver library is covered in detail in Chapter 6, Name Service and Resolver Configuration.
The hosts file contains one entry per line, consisting of an IP address, a hostname, and an optional list of aliases for the hostname. The fields are separated by spaces or tabs, and the address field must begin in the first column. Anything following a hash sign (#) is regarded as a comment and is ignored. Hostnames can be either fully qualified or relative to the local domain. For vale, you would usually enter the fully qualified name, vale.vbrew.com, and vale by itself in the hosts file, so that it is known by both its official name and the shorter local name.
This is an example how a hosts file at the Virtual Brewery might look. Two special names are included, vlager-if1 and vlager-if2, which give the addresses for both interfaces used on vlager:
#
# Hosts file for Virtual Brewery/Virtual Winery
#
# IP FQDN aliases
#
127.0.0.1 localhost
#
172.16.1.1 vlager.vbrew.com vlager vlager-if1
172.16.1.2 vstout.vbrew.com vstout
172.16.1.3 vale.vbrew.com vale
#
172.16.2.1 vlager-if2
172.16.2.2 vbeaujolais.vbrew.com vbeaujolais
172.16.2.3 vbardolino.vbrew.com vbardolino
172.16.2.4 vchianti.vbrew.com vchianti
Just as with a host's IP address, you should sometimes use a symbolic name for network numbers, too. Read More..
IP Addresses
As mentioned in Chapter 1, Introduction to Networking, the IP networking protocol understands addresses as 32-bit numbers. Each machine must be assigned a number unique to the networking environment.[1] If you are running a local network that does not have TCP/IP traffic with other networks, you may assign these numbers according to your personal preferences. There are some IP address ranges that have been reserved for such private networks. These ranges are listed in Table 2.1. However, for sites on the Internet, numbers are assigned by a central authority, the Network Information Center (NIC).
[1] The version of the Internet Protocol most frequently used on the Internet is Version 4. A lot of effort has been expended in designing a replacement called IP Version 6. IPv6 uses a different addressing scheme and larger addresses. Linux has an implementation of IPv6, but it isn't ready to document it in this book yet. The Linux kernel
support for IPv6 is good, but a large number of network applications need to be modified to support it as well. Stay tuned.
[2] Frequently, IP addresses will be assigned to you by the provider from whom you buy your IP connectivity. However, you may also apply to the NIC directly for an IP address for your network by sending email to hostmaster@internic.net, or by using the form at
http://www.internic.net/.
IP addresses are split up into four eight-bit numbers called octets for readability. For example, quark.physics.groucho.edu has an IP address of 0x954C0C04, which is written as 149.76.12.4. This format is often referred to as dotted quad notation. Another reason for this notation is that IP addresses are split into a network number, which is contained in the leading octets, and a host number, which is the remainder. When applying to the NIC for IP addresses, you are not assigned an address for each single host you plan to use. Instead, you are given a network number and allowed to assign all valid IP addresses within this range to hosts on your network according to your preferences.
The size of the host part depends on the size of the network. To accommodate different needs, several classes of networks, defining different places to split IP addresses, have been defined. The class networks are described here:
Class A
Class A comprises networks 1.0.0.0 through 127.0.0.0. The network number is contained in the first octet. This class provides for a 24-bit host part,allowing roughly 1.6 million hosts per network.
Class B
Class B contains networks 128.0.0.0 through 191.255.0.0; the network number is in the first two octets. This class allows for 16,320 nets with 65,024 hosts each.
Class C
Class C networks range from 192.0.0.0 through 223.255.255.0, with the network number contained in the first three octets. This class allows for nearly 2 million networks with up to 254 hosts.
Classes D, E, and F
Addresses falling into the range of 224.0.0.0 through 254.0.0.0 are either experimental or are reserved for special purpose use and don't specify any network. IP Multicast, which is a service that allows material to be transmitted to many points on an internet at one time, has been assigned addresses from within this range.
If we go back to the example in Chapter 1, we find that 149.76.12.4, the address of quark, refers to host 12.4 on the class B network 149.76.0.0. You may have noticed that not all possible values in the previous list were allowed for each octet in the host part. This is because octets 0 and 255 are reserved for special purposes. An address where all host part bits are 0 refers to the network, and an address where all bits of the host part are 1 is called a broadcast address. This refers to all hosts on the specified network simultaneously. Thus, 149.76.255.255 is not a valid host address, but refers to all hosts on network 149.76.0.0.
A number of network addresses are reserved for special purposes. 0.0.0.0 and 127.0.0.0 are two such addresses. The first is called the default route, and the latter is the loopback address. The default route has to do with the way the IP routes datagrams. Network 127.0.0.0 is reserved for IP traffic local to your host. Usually, address 127.0.0.1 will be assigned to a special interface on your host, the loopback interface, which acts like a closed circuit. Any IP packet handed to this interface from TCP or UDP will be returned to them as if it had just arrived from some network. This allows you to develop and test networking software without ever using a "real" network. The loopback network also allows you to use networking software on a standalone host. This may not be as uncommon as it sounds; for instance, many UUCP sites don't have IP connectivity at all, but still want to run the INN news system. For proper operation on Linux, INN requires the loopback interface.
Some address ranges from each of the network classes have been set aside and designated "reserved" or "private" address ranges. These addresses are reserved for use by private networks and are not routed on the Internet. They are commonly used by organizations building their own intranet, but even small networks often find them useful. Read More..
[1] The version of the Internet Protocol most frequently used on the Internet is Version 4. A lot of effort has been expended in designing a replacement called IP Version 6. IPv6 uses a different addressing scheme and larger addresses. Linux has an implementation of IPv6, but it isn't ready to document it in this book yet. The Linux kernel
support for IPv6 is good, but a large number of network applications need to be modified to support it as well. Stay tuned.
[2] Frequently, IP addresses will be assigned to you by the provider from whom you buy your IP connectivity. However, you may also apply to the NIC directly for an IP address for your network by sending email to hostmaster@internic.net, or by using the form at
http://www.internic.net/.
IP addresses are split up into four eight-bit numbers called octets for readability. For example, quark.physics.groucho.edu has an IP address of 0x954C0C04, which is written as 149.76.12.4. This format is often referred to as dotted quad notation. Another reason for this notation is that IP addresses are split into a network number, which is contained in the leading octets, and a host number, which is the remainder. When applying to the NIC for IP addresses, you are not assigned an address for each single host you plan to use. Instead, you are given a network number and allowed to assign all valid IP addresses within this range to hosts on your network according to your preferences.
The size of the host part depends on the size of the network. To accommodate different needs, several classes of networks, defining different places to split IP addresses, have been defined. The class networks are described here:
Class A
Class A comprises networks 1.0.0.0 through 127.0.0.0. The network number is contained in the first octet. This class provides for a 24-bit host part,allowing roughly 1.6 million hosts per network.
Class B
Class B contains networks 128.0.0.0 through 191.255.0.0; the network number is in the first two octets. This class allows for 16,320 nets with 65,024 hosts each.
Class C
Class C networks range from 192.0.0.0 through 223.255.255.0, with the network number contained in the first three octets. This class allows for nearly 2 million networks with up to 254 hosts.
Classes D, E, and F
Addresses falling into the range of 224.0.0.0 through 254.0.0.0 are either experimental or are reserved for special purpose use and don't specify any network. IP Multicast, which is a service that allows material to be transmitted to many points on an internet at one time, has been assigned addresses from within this range.
If we go back to the example in Chapter 1, we find that 149.76.12.4, the address of quark, refers to host 12.4 on the class B network 149.76.0.0. You may have noticed that not all possible values in the previous list were allowed for each octet in the host part. This is because octets 0 and 255 are reserved for special purposes. An address where all host part bits are 0 refers to the network, and an address where all bits of the host part are 1 is called a broadcast address. This refers to all hosts on the specified network simultaneously. Thus, 149.76.255.255 is not a valid host address, but refers to all hosts on network 149.76.0.0.
A number of network addresses are reserved for special purposes. 0.0.0.0 and 127.0.0.0 are two such addresses. The first is called the default route, and the latter is the loopback address. The default route has to do with the way the IP routes datagrams. Network 127.0.0.0 is reserved for IP traffic local to your host. Usually, address 127.0.0.1 will be assigned to a special interface on your host, the loopback interface, which acts like a closed circuit. Any IP packet handed to this interface from TCP or UDP will be returned to them as if it had just arrived from some network. This allows you to develop and test networking software without ever using a "real" network. The loopback network also allows you to use networking software on a standalone host. This may not be as uncommon as it sounds; for instance, many UUCP sites don't have IP connectivity at all, but still want to run the INN news system. For proper operation on Linux, INN requires the loopback interface.
Some address ranges from each of the network classes have been set aside and designated "reserved" or "private" address ranges. These addresses are reserved for use by private networks and are not routed on the Internet. They are commonly used by organizations building their own intranet, but even small networks often find them useful. Read More..
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