Foundations of Networking

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Class E Addresses

Addresses in the range 224.0.0.0 to 254.255.255.255 are termed class E addresses. The first octet of these addresses begins with the bits 1111. These addresses are reserved for future additions to the IP addressing scheme. These future additions might or might not come to fruition with the advent of IPv6, which will be discussed in later chapters.

In most networks, the IP addresses assigned to each have been broken into parts that logically relate to different areas. For example, part of an IP address identifies a particular network, part identifies a subnet (that is, subnetwork), and part identifies a specific host within that subnetwork (that is, subnet).

The following three blocks of IP address space for private networks have been reserved according to RFC 1597: Address Allocation for Private Internets.

•  10.0.0.0-10.255.255.255. Single class A network numbers

•  172.16.0.0-172.31.255.255. Contiguous class B network numbers

•  192.168.0.0-192.168.255.255. Contiguous class C network numbers

How IP Addresses Are Used

Routers examine the most significant or left-most bit of the first octet when determining the class of a network address. This technique of reading IP addresses (also known as the first octet rule) is discussed further as the different classes of addresses are defined.

Table 2-1 provides a variety of quick reference information relating to the different IP address classes. Note that in the format column, N=Network number and H=Host number. Also, for Class A addresses, one address is reserved for the broadcast address, and one address is reserved for the network.

Table 2-1 Quick IP Address Reference Information

IP Address Class	Format	Purpose	High-Order Bit(s)	Address Range	No. Bits Network/Host	Maximum Hosts
A	N.H.H.H	Few large organizations	0	1.0.0.0-126.0.0.0	7/24	16,777,2 14 (2 [24]- 2)
B	N.N.H.H	Medium-sized organizations	1,0	128.1.0.0-191.254.0.0	14/16	65,543 (2[16] -2)
C	N.N.N.H	Relatively small organizations	1,1,0	192.0.1.0-223.255.254.0	22/8	254 (2[8] -2)
D	N/A	Multi-cast groups (RFC 1112)	1,1,1,0	224.0.0.0-239.255.255.255	N/A (not N/A for commercial use)	N/A
E	N/A	Experimental	1,1,1,1	240.0.0.0-254.255.255.255	N/A	N/A

Tables 2-2 through 2-4 list the actual number of hosts and subnets for Class A, B, and C IP addresses. For the subnets and hosts, all zeroes and all ones are excluded.

Table 2-2 Host/Subnet Quantities for Class A IP Addresses

Number of bits	Mask	Effective Subnets	Effective Hosts
2	255.192.0.0	2	4,194,302
3	255.224.0.0	6	2,097,150
4	255.240.0.0	14	1,048,574
5	255.248.0.0	30	524,286
6	255.252.0.0	62	262,142
7	255.254.0.0	126	131,070
8	255.255.0.0	254	65,536
9	255.255.128.0	510	32,766
10	255.255.192.0	1,022	16,382
11	255.255.224.0	2,046	8,190
12	255.255.240.0	4,094	4,094
13	255.255.248.0	8,190	2,046
14	255.255.252.0	16,382	1,022
15	255.255.254.0	32,766	5,10
16	255.255.255.0	65,536	254
17	255.255.255.128	131,070	126
18	255.255.255.192	262,142	62
19	255.255.255.224	524,286	30
20	255.255.255.240	1,048,574	14
21	255.255.255.248	2,097,150	6
22	255.255.255.252	4,194,302	2

Table 2-3 lists the actual number of hosts and subnets for Class B IP addresses.

Table 2-3 Host/Subnet Quantities for Class B IP Addresses

Number of bits	Mask	Effective Subnets	Effective Hosts
2	255.255.192.0	2	16,382
3	255.255.224.0	6	8,190
4	255.255.240.0	14	4,094
5	255.255.248.0	30	2,046
6	255.255.252.0	62	1,022
7	255.255.254.0	126	510
8	255.255.255.0	254	254
9	255.255.255.128	510	126
10	255.255.255.192	1,022	62
11	255.255.255.224	2,046	30
12	255.255.255.240	4,094	14
13	255.255.255.248	8,190	6
14	255.255.255.252	16,382	2

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