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Debian GNU/Linux Reference Guide
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Debian Reference
Chapter 10 - Network configuration

This chapter focuses on network administration in Debian. For a general introduction to GNU/Linux networking read the Net-HOWTO.

In order for a Debian host to be able to access the Internet its network interfaces need to be properly configured.

The first requirement is kernel support for the devices. Examples of such devices are: Ethernet cards, Wi-Fi cards, and modems. To obtain this support you may need to recompile the kernel or add modules to it as described in The Linux kernel under Debian, Chapter 7.

Configuration of network devices is explained below. The information in this chapter was updated for Sarge. Much of it does not apply to earlier releases.

10.1 Basics of IP networking

A Debian host may have several interfaces each with a different Internet Protocol (IP) address. Interfaces may be of several different types, including:

  • Loopback: lo

  • Ethernet: eth0, eth1, ...

  • Wi-Fi: wlan0, wlan1, wifi0, ... [51]

  • Token Ring: tr0, tr1, ...

  • PPP: ppp0, ppp1, ...

There is a wide range of other network devices available, including SLIP, PLIP (serial and parallel line IP), "shaper" devices for controlling the traffic on certain interfaces, frame relay, AX.25, X.25, ARCnet, and LocalTalk.

Every network interface connected directly to the Internet (or to any IP-based network) is identified by a unique 32 bit IP address. [52] The IP address can be divided into the part that addresses the network and the part that addresses the host. If you take an IP address, set to 1 the bits that are part of the network address and set to 0 the bits that are part of the host address then you get the so-called netmask of the network.

Traditionally, IP networks were grouped into classes whose net address parts were 8, 16 or 24 bits in length. This system was inflexible and wasted many IP addresses, so today IPv4 networks are allocated with network address parts of varying length.

               IP addresses                   net mask      length
     Class A     -     =  /8
     Class B   -   = /16
     Class C   - = /24

IP addresses not in these ranges are used for special purposes.

There are address ranges in each class reserved for use on local area networks (LANs). These addresses are guaranteed not to conflict with any addresses on the Internet proper. (By the same token, if one of these addresses is assigned to a host then that host must not access the Internet directly but must access it through a gateway that acts as a proxy for individual services or else does Network Address Translation.) These address ranges are given in the following table along with the number of ranges in each class.

               network addresses            length  how many
     Class A   10.x.x.x                     /8      1
     Class B   172.16.x.x -  172.31.x.x     /16     16
     Class C   192.168.0.x - 192.168.255.x  /24     256

The first address in an IP network is the address of the network itself. The last address is the broadcast address for the network. [53] All other addresses may be allocated to hosts on the network. Of these, the first or the last address is usually allocated to the Internet gateway for the network.

The routing table contains the kernel's information on how to send IP packets to their destinations. Here is a sample routing table printout for a Debian host on a local area network (LAN) with IP address 192.168.50.x/24. Host (also on the LAN) is a router for the corporate network 172.20.x.x/16 and host (also on the LAN) is a router for the Internet at large.

     # route
     Kernel IP routing table
     Destination   Gateway        Genmask       Flags Metric Ref Use Iface     *         U     0      0     2 lo  *     U     0      0   137 eth0   UG    1      0     7 eth0
     default       UG    1      0    36 eth0
  • The first line after the heading says that traffic destined for network 127.x.x.x will be routed through lo, the loopback interface.

  • The second line says that traffic destined for hosts on the LAN will be routed through eth0.

  • The third line says that traffic destined for the corporate network will be routed toward gateway also through eth0.

  • The fourth line says that traffic destined for the Internet at large will be routed toward gateway also through eth0.

IP addresses in the table may also appear as names that are obtained by looking up addresses in /etc/networks or by using the C Library resolver.

In addition to routing, the kernel can perform network address translation, traffic shaping and filtering.

See the Net-HOWTO and other networking HOWTOs for more background information.

Debian GNU/Linux Reference Guide
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  Published under the terms of the GNU General Public License Design by Interspire