GE GFK-1004B Manuel D’Utilisation

TCP/IP Ethernet Communications User’s Manual – January 1996

G-2

GFK-1004B

One can distinguish the class of an IP address from the first integer in its dotted-decimal
IP address as follows.

ÁÁÁÁÁÁÁÁ

Range of first integer

ÁÁÁÁÁ

Class

ÁÁÁÁÁÁÁÁ

0 - 127

ÁÁÁÁÁ

ÁÁÁÁÁÁÁÁ

128 - 191

ÁÁÁÁÁ

ÁÁÁÁÁÁÁÁ

192 - 223

ÁÁÁÁÁ

ÁÁÁÁÁÁÁÁ

224 - 255

ÁÁÁÁÁ

reserved

Gateways

Gateways (also known as routers) connect individual physical networks into a system of
networks. When a host needs to communicate with a host on another network, a
gateway transfers the data between the two networks.

The following figure shows gateway G connecting Network 1 with Network 2.

a45405

128.1.0.1

Network 1

128.2.0.1

ÎÎÎ

128.2.0.2

ÎÎÎ

128.2.0.3

ÎÎÎ

Network 2

128.1.0.2

Figure G-2. Connecting Two Networks with a Gateway

When host B with IP address 128.2.0.1 communicates with host C, it knows from C’s IP
address that C is on the same network. In our Ethernet environment it can then resolve
C’s IP address to a MAC address (via ARP) and communicate with C directly.

When host B communicates with host A, it knows from A’s IP address that A is on
another network (the netids are different). In order to send data to A, B must have the IP
address of the gateway connecting the two networks. In this example, the gateway’s IP
address on Network 2 is 128.2.0.3.

Note that the gateway has two IP addresses (128.1.0.2 and 128.2.0.3). The first must be
used by hosts on Network 1 and the second must be used by hosts on Network 2. To be
usable, a host’s gateway must be addressed using an IP address with a netid matching its
own.