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TCP/IP Ethernet Communications User’s Manual – January 1996G-2
GFK-1004B
One can distinguish the class of an IP address from the first integer in its dotted-decimal
IP address as follows.
Range of first integer Class
0 - 127 A
128 - 191 B
192 - 223 C
224 - 255 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
128.2.0.1 128.2.0.2
128.2.0.3
Network 2
128.1.0.2
A
BC
G
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.