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G-3
GFK-1004B
Appendix G Advanced Information About IP and MAC Addresses
Subnets
Subnet addressing is an extension of the IP address scheme that allows a site to use a
single netid for multiple physical networks. Routing outside of the site continues as
usual by dividing the IP address into a netid and a hostid via the class. Inside a site the
subnet mask is used to re-divide the IP address into a custom netid portion and hostid
portion.
Consider taking Network 2 (a Class B network) in the previous example and adding
another physical network. Selecting the following subnet mask would add two
additional netid bits allowing for four physical networks:
11111111 11111111 11000000 00000000 = 255.255.192.0
In effect, two bits of the Class B hostid have been used to extend the netid.
The new configuration would be:
a45406
128.1.0.1
A
Network 1
128.2.64.1
B
128.2.64.2
C
128.2.64.3
G
Network 2.1
128.2.128.1
D
128.2.128.2
E
128.2.128.3
G2
Network 2.2
128.1.0.2
Figure G-3. Network Configuration Using a Subnet Mask
A second network with Hosts D and E has been added. Gateway G2 connects Network
2.1 with Network 2.2. Hosts D and E will use Gateway G2 to communicate with hosts
not on Network 2.2. Hosts B and C will use Gateway G to communicate with hosts not
on Network 2.1. When B is communicating with D, G (the configured Gateway for B)
will route the data from B to D through G2.