GE GFK-1004B user manual Subnets, Figure G-3. Network Configuration Using a Subnet Mask

G

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:

A

128.1.0.1

Network 1

a45406

128.1.0.2

G

B C

128.2.64.3

128.2.64.1128.2.64.2

		Network 2.1
					G2
D			E
						128.2.128.3
	128.2.128.1			128.2.128.2
		Network 2.2

Figure G-3. Network Configuration Using a Subnet Mask

A second network with Hosts D and E has been added. Gateway G2 connects Network

2.1with 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.

GFK-1004B Appendix G Advanced Information About IP and MAC Addresses G-3