Lucent Technologies Release 8.2 manual No. Binary Subnet, Class-C subnets, Decimal, Bit subnets

IP Addressing						1 Networking Overview
Class-C subnets	The following table shows that Class-C IP addresses can have 5 subnetting schemes,
	each with a different number of subnets per network. The first and last subnet, formed
	by using 1 and 7 bits respectively, are unusable because they result in either the
	subnet ID or the host ID having all zeros or all ones.
	No.	No. of	No. of		Binary
				No. of	Subnet	Decimal
	Sub-	Usable	Hosts				Class C
					ID
	net	Subnets	per	Usable IP		Subnet	Subnet Masks
				Addresses	(4th	ID
	bits	per NW	Subnet
					Octet)
	1	0	126	0	10000000	128	255.255.255.128
	2	2	62	124	11000000	192	255.255.255.192
	3	6	30	180	11100000	224	255.255.255.224
	4	14	14	196	11110000	240	225.225.225.240
	5	30	6	180	11111000	248	255.255.255.248
	6	62	2	124	11111100	252	255.255.255.252
	7	126	0	0	11111110	254	255.255.255.254

3-bit subnets

As an example, the third row of the table shows the results of using 3 bits for the subnet ID. Three bits are “borrowed” from the host ID leaving 5 bits for the host IDs. The number of subnets that can be defined with three bits is 23 = 8 (000, 001, 010, 011, 100, 101, 110, 111). Of these, only 6 are usable (all ones and all zeros are not usable). The remaining 5 bits are used for the host IDs. Of these, 25 – 2 = 30 are usable. As shown in columns 2–4 (row 3), by using 3 bits for subnetting, a Class C network can be divided into 6 subnets with 30 host IDs in each subnet for a total of 6 X 30 = 180 usable IP addresses.

Subnet mask

The subnet mask is defined as follows. The subnet bits “borrowed” from the host ID are the highest-order bits in the octet of the host ID. The 5th and 6th columns of the table show the binary and decimal subnet IDs, formed by using the subnet bits as the highest-order bits in an octet. For example, in the third row of the table, the binary bit pattern is 11100000, which is decimal 224. This is the highest number that can be formed with the 3 high-order bits in the octet. The subnet mask is formed by putting this number in the 4th octet of the default subnet mask (shown in the last column of the table).

The mask, 255.255.255.224, corresponds to a bit pattern of 27 ones followed by 5 zeros. This mask would be used to check that two IP addresses are on the same or different subnets by comparing the first 27 binary digits of the two addresses. If the first 27 binary digits are the same, the two addresses are on the same subnet.

		Administration for Network Connectivity
18	CID: 77730	555-233-504 — Issue 1 — April 2000