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IP Classless Addressing Block Sizes and "Classful" Network Equivalents
Since CIDR allows us to divide IP addresses into network ID and host ID along any bit boundary, it permits the creation of dozens different sizes of networks. As with subnetting, the size of network is a trade-off between the number of bits used for the network ID and the number used for the host ID. Unlike conventional subnetting, where a single choice is made for all subnets, CIDR allows many levels of hierarchical division of the Internet, so many sizes of networks exist simultaneously. Larger networks are created and subdivided into smaller ones, as we saw in the previous topic.
Since many people are used to looking at IP address blocks in terms of their “classful” sizes, it is common to express CIDR address blocks in terms of their “classful” equivalents. First of all, it should be simple at this point to see that a CIDR “/8” network is equal in size to a Class A network; a “/16” is equivalent to a Class B; a “/24” is equivalent to a Class C. This is of course because Class A networks use 8 bits for the network ID, Class Bs use 16, and Class Cs use 24. However, remember that these CIDR equivalents do not need to have any particular ranges for their first octets as in the “classful” scheme.
Each time we reduce the prefix length, we are defining a network about double the size of the one with the higher number, since we have increased the number of bits in the host ID by one. So, a “/15” network is equal in size to two “/16”s.
Table 55 shows each of the possible theoretical ways to divide the 32 bits of an IP address into network ID and host ID bits under CIDR. For each, I have shown the number of hosts in each network, and the way a network of each size is represented in both slash notation and as a conventional subnet mask. I have also shown the equivalent number of Class A, Class B and Class C networks for each.
A few things to remember in looking at this table:
- Some of the entries shown are more theoretical
than practical, and are included merely for completeness. This is particularly
the case with the larger networks. For example, I doubt anyone ever
actually works with a /1 or /2 size CIDR network; there would only be
two of the former and four of the latter encompassing the entire IP
address space! Most of the time you will be working with smaller networks,
/16 and below.
- Under normal circumstances, you cannot have a
/31 or /32 CIDR network since they would have zero valid host IDs. (There
is a special case: /31 networks can be used for point-to-point links,
where it is obvious who the intended recipient is of each transmission,
and where broadcasts are not necessary. This is described in RFC 3021.)
- In the columns showing the number of equivalent
Class A, B and C networks I have only shown numbers in the range of
1/256th to 256 for simplicity. Obviously, a /6 network, in addition
to being equal in size to 4 Class A networks, also equals 1,024 Class
Bs and 262,144 Class C networks, but few people would bother referring
to a /6 as being 262,144 Class Cs.
Table 55: CIDR Address Blocks and “Classful” Address Equivalents
# of Bits For Network ID
# of Bits For Host ID
# of Hosts Per Network
Prefix Length in Slash Notation
Equivalent Subnet Mask
# of Equivalent “Classful” Addressing Networks
Class A
Class B
Class C
1
31
2,147,483,646
/1
128.0.0.0
128
—
—
2
30
1,073,741,822
/2
192.0.0.0
64
—
—
3
29
536,870,910
/3
224.0.0.0
32
—
—
4
28
268,435,454
/4
240.0.0.0
16
—
—
5
27
134,217,726
/5
248.0.0.0
8
—
—
6
26
67,108,862
/6
252.0.0.0
4
—
—
7
25
33,554,430
/7
254.0.0.0
2
—
—
8
24
16,777,214
/8
255.0.0.0
1
256
—
9
23
8,388,606
/9
255.128.0.0
1/2
128
—
10
22
4,194,302
/10
255.192.0.0
1/4
64
—
11
21
2,097,150
/11
255.224.0.0
1/8
32
—
12
20
1,048,574
/12
255.240.0.0
1/16
16
—
13
19
524,286
/13
255.248.0.0
1/32
8
—
14
18
262,142
/14
255.252.0.0
1/64
4
—
15
17
131,070
/15
255.254.0.0
1/128
2
—
16
16
65,534
/16
255.255.0.0
1/256
1
256
17
15
32,766
/17
255.255.128.0
—
1/2
128
18
14
16,382
/18
255.255.192.0
—
1/4
64
19
13
8,190
/19
255.255.224.0
—
1/8
32
20
12
4,094
/20
255.255.240.0
—
1/16
16
21
11
2,046
/21
255.255.248.0
—
1/32
8
22
10
1,022
/22
255.255.252.0
—
1/64
4
23
9
510
/23
255.255.254.0
—
1/128
2
24
8
254
/24
255.255.255.0
—
1/256
1
25
7
126
/25
255.255.255.128
—
—
1/2
26
6
62
/26
255.255.255.192
—
—
1/4
27
5
30
/27
255.255.255.224
—
—
1/8
28
4
14
/28
255.255.255.240
—
—
1/16
29
3
6
/29
255.255.255.248
—
—
1/32
30
2
2
/30
255.255.255.252
—
—
1/64
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Version 3.0 - Version Date: September 20, 2005
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