Lucent Technologies Release 8.2 128, Class a, 50% Class B, 25% Class C, 12.5% Class D, Class E

IP Addressing 1 Networking Overview

Conversion between binary and decimal

IP Address Classes

Conversion from binary to decimal notation is accomplished by adding the powers of

2 corresponding to the 1’s positions in each byte:

		27 =	26 =	25 =	24 =	23 =	22 =	21 =	20 =
		128	64	32	16	8	4	2	1
194	=	1	1	0	0	0	0	1	0
13	=	0	0	0	0	1	1	0	1
219	=	1	1	0	1	1	0	1	1
7	=	0	0	0	0	0	1	1	1

The IP address space (232 or about 4.3 billion addresses) has been divided into five groups, Classes A–E, to accommodate the need for different network sizes. Each class has a different allocation of bits between the network and host IDs. The classes are identified by a fixed pattern of leading bits.

In Class A addresses, the first (leftmost) bit is always 0. So Class A IP addresses have 7 bits to define network IDs; 7 bits can define a total of 128 (0-->127) Class A networks. The remaining 24 bits of a Class A IP address are used to define host IDs. So for each of the 126 networks, there are 224 or 16,777,216 possible hosts.

The following table shows how IP addresses are the allocated among the five classes.

		Octet 1	Octet 2	Octet 3	Octet 4
Class A	0	Network ID		Host ID
50%
Class B	1 0		Network ID	Host ID
25%
Class C	1 1 0		Network ID		Host ID
12.5%
Class D	1 1 1 0		Reserved for Multicast addresses
6.5%
Class E	1 1 1 1		Reserved for future use
6.5%

Address classes A, B, and C cover 87.5% of the address space. These addresses are assigned by the ISP or the Internet Assigned Number Authority (IANA) to organizations for their exclusive use. The remaining 12.5% of addresses, designated classes D and E, are reserved for special purposes.

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