G |
One can distinguish the class of an IP address from the first integer in its
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.
A
128.1.0.1
Network 1
a45405
128.1.0.2
B
Î128.2.0.1
C
Î128.2.0.2
G
Î128.2.0.3
Network 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.
TCP/IP Ethernet Communications User's Manual ± January 1996 |
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