Datasheet

“main” (Installation and Administration) — 2004/6/25 — 13:29 — page 420 — #446

However, the economical ethernet is not suitable for covering larger dis-

tances. You must transfer the IP packets to another hardware (such as FDDI

or ISDN). Devices for this transfer are called routers or gateways. A Linux

machine can carry out this task. The respective option is referred to as

ip_forwarding.

If a gateway has been conﬁgured, the IP packet is sent to the appropriate

gateway. This then attempts to forward the packet in the same manner —

from host to host — until it reaches the destination host or the packet’s TTL

(time to live) expires.

Table 21.2: Speciﬁc Addresses

Address Type Description

Base network address This is the netmask AND any address in the net-

work, as shown in Table 21.2 on the page before

under Result. This address cannot be assigned

to any hosts.

Broadcast address This basically says, “Access all hosts in this

subnetwork.” To generate this, the netmask is

inverted in binary form and linked to the base

network address with a logical OR. The above

example therefore results in 192.168.0.255. This

address cannot be assigned to any hosts.

Local host The address 127.0.0.1 is strictly assigned to

the “loopback device” on each host. A connec-

tion can be set up to your own machine with this

address.

As IP addresses must be unique all over the world, you cannot just come

up with your own random addresses. There are three address domains to

use to set up a private IP-based network. With these, you cannot set up any

connections to the rest of the Internet, unless you apply certain tricks, be-

cause these addresses cannot be transmitted over the Internet. These ad-

dress domains are speciﬁed in RFC 1597 and listed in Table 21.3 on the fac-

ing page.

420 21.1. TCP/IP — The Protocol Used by Linux