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Subnetting allows creating multiple logical networks from a single address block. Because a router connects these networks, each interface on a router must have a unique network ID. Every node on that link is on the same network.

You create the subnets by reassigning one or more of the host bits as network bits. This is done by extending the prefix to “borrow” some of the bits from the host portion of the address to create additional network bits. The more host bits borrowed, the more subnets that can be defined. For each bit borrowed, you double the number of subnetworks avail-able. For example, if you borrow 1 bit, you can define two subnets. If you borrow 2 bits, you can have four subnets.

However, with each bit you borrow, you have fewer host bits to define the host addresses in each subnet. Therefore, there are fewer host addresses available per subnet. Additionally, because you have two addresses for each network—network address and broadcast address—that cannot be assigned to hosts, the total number of hosts in the entire network decreases.

Creating Two Subnets

Router A in Figure 6-18 has two interfaces to interconnect two networks. Given an address block of 192.168.1.0 /24, you will create two subnets. You borrow 1 bit from the host por-tion by using a subnet mask of 255.255.255.128, instead of the original 255.255.255.0 mask. This makes the most significant bit in the last octet a network bit instead of a host bit. This bit is used to distinguish between the two subnets. For one of the subnets, this bit is a 0, and for the other subnet, this bit is a 1. The information for these two subnets is shown in Table 6-15.

Figure 6-18     Borrowing a Bit to Create Two Subnets

	1					Only one network address is available.
Router A					1 192.168.1.0 (/24)		Address:	11000000.10101000.00000001.00000000
2					255.255.255.0		Mask:	11111111.11111111.11111111.00000000
								Network portion of the address.
								Borrow a bit from
								the host portion.
1
						With subnetting, two network addresses are available.
Router A					1 192.168.1.0 (/25)		Address:	11000000.10101000.00000001.00000000
2					255.255.255.128		Mask:	11111111.11111111.11111111.10000000
					2 192.168.1.128 (/25)		Address:	11000000.10101000.00000001.10000000
					255.255.255.128		Mask:	11111111.11111111.11111111.10000000

Increase the network

portion of the address.