213

Table 6-15       Subnets for the 192.168.1.0 /24 Network with 1 Borrowed Bit

Subnet	Network Address	Host Range	Broadcast Address
1	192.168.1.0/25	192.168.1.1–192.168.1.126	192.168.1.127
2	192.168.1.128/25	192.168.1.129–192.168.1.254	192.168.1.255

Use this formula to calculate the number of subnets:

2ⁿ, where n = the number of bits borrowed

In the example of Figure 6-18 and Table 6-15, the calculation is

2¹ = 2 subnets

For each subnet, examine the last octet of the subnet address in binary. The values in these octets for the two networks are

Subnet 1: 00000000 = 0

Subnet 2: 10000000 = 128

To calculate the number of hosts per network, you use the formula of 2ⁿ – 2, where n = the number of bits left for hosts.

Applying this formula to the two-subnet example in Figure 6-18 and Table 6-15, 2⁷ – 2 = 126 shows that each of these subnets can have 126 hosts.

Creating Three Subnets

Beginning with the previous example, consider an internetwork that requires three subnets. The network in Figure 6-19 starts with the same 192.168.1.0 /24 address block. Borrowing a single bit would only provide two subnets. To provide more networks, you change the subnet mask to 255.255.255.192 and borrow 2 bits. These 2 bits will provide four subnets. These networks are shown in Table 6-16. The calculations follow.

Figure 6-19     Borrowing 2 Bits to Create Subnets

					–	192.168.1.0 (/24)	Address:	11000000.10101000.00000001.00000000
0
						255.255.255.0	Mask:	11111111.11111111.11111111.00000000
1	Router A				0	192.168.1.0 (/26)	Address:	11000000.10101000.00000001.00000000
						255.255.255.192	Mask:	11111111.11111111.11111111.11000000

²                    ₁ 192.168.1.64 (/26)    Address:  11000000.10101000.00000001.01000000

255.255.255.192Mask:11111111.11111111.11111111.11000000

₂ 192.168.1.128 (/26)  Address: 11000000.10101000.00000001.10000000

255.255.255.192     Mask:     11111111.11111111.11111111.11000000

₃ 192.168.1.192 (/26)                                          Address:            11000000.10101000.00000001.11000000

255.255.255.192     Mask:     11111111.11111111.11111111.11000000

Two bits are borrowed

to provide four subnets.

A 1 in these positions in the mask means that these values are part of the network address.

Unused address in