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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OSPF Network Design Solutions
OSPF Network Design Solutions
ISBN: 1578700469
EAN: 2147483647
Year: 1998
Pages: 200
Authors: Tom Thomas

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