Subnetting
Although the conceptual notion of subnetting by utilizing host bits is straightforward, the actual mechanics of subnetting are a bit more complicated. Subnetting requires a three step procedure:
Determine the number of host bits to be used for the subnetting.
Enumerate the new subnetted network IDs.
Enumerate the IP addresses for each new subnetted network ID.
Step 1: Determining the Number of Host Bits
The number of host bits being used for subnetting determines the possible number of subnets and hosts per subnet. Before you choose the number of host bits, you should have a good idea of the number of subnets and hosts you will have in the future. Using more bits for the subnet mask than required saves you the time of reassigning IP addresses in the future.
The more host bits that are used, the more subnets (subnetted network IDs) you can have — but with fewer hosts. Using too many host bits allows for growth in the number of subnets but limits the growth in the number of hosts. Using too few hosts allows for growth in the number of hosts but limits the growth in the number of subnets.
In practice, network administrators define a maximum number of nodes they want on a single network. Recall that all nodes on a single network share all the same broadcast traffic; they reside in the same broadcast domain. Therefore, growth in the number of subnets is favored over growth in the number of hosts per subnet.
Follow these guidelines to determine the number of host bits to use for subnetting.
Determine how many subnets you need now and will need in the future. Each physical network is a subnet. WAN connections can also count as subnets depending on whether your routers support unnumbered connections.
Use additional bits for the subnet mask if:
You will never require as many hosts per subnet as allowed by the remaining bits.
The number of subnets will increase in the future, requiring additional host bits.
To determine the desired subnetting scheme, start with an existing network ID to be subnetted. The network ID to be subnetted can be a class-based network ID, a subnetted network ID, or a supernet. The existing network ID contains a series of network ID bits that are fixed and a series of host ID bits that are variable. Based on your requirements for the number of subnets and the number of hosts per subnet, choose a specific number of host bits to be used for the subnetting.
Table 1.6 shows the subnetting of a class A network ID. Based on a required number of subnets, and a maximum number of hosts per subnet, a subnetting scheme can be chosen.
Table 1.6 Subnetting a Class A Network ID
Required Number of Subnets
Number of Subnet Bits
Subnet Mask
Number of Hosts per Subnet
1-2
1
255.128.0.0 or /9
8,388,606
3-4
2
255.192.0.0 or /10
4,194,302
5-8
3
255.224.0.0 or /11
2,097,150
9-16
4
255.240.0.0 or /12
1,048,574
17-32
5
255.248.0.0 or /13
524,286
33-64
6
255.252.0.0 or /14
262,142
65-128
7
255.254.0.0 or /15
131,070
129-256
8
255.255.0.0 or /16
65,534
257-512
9
255.255.128.0 or /17
32,766
513-1,024
10
255.255.192.0 or /18
16,382
1,025-2,048
11
255.255.224.0 or /19
8,190
2,049-4,096
12
255.255.240.0 or /20
4,094
4,097-8,192
13
255.255.248.0 or /21
2,046
8,193-16,384
14
255.255.252.0 or /22
1,022
16,385-32,768
15
255.255.254.0 or /23
510
32,769-65,536
16
255.255.255.0 or /24
254
65,537-131,072
17
255.255.255.128 or /25
126
131,073-262,144
18
255.255.255.192 or /26
62
262,145-524,288
19
255.255.255.224 or /27
30
524,289-1,048,576
20
255.255.255.240 or /28
14
1,048,577-2,097,152
21
255.255.255.248 or /29
6
2,097,153-4,194,304
22
255.255.255.252 or /30
2
Table 1.7 shows the subnetting of a class B network ID.
Table 1.7 Subnetting a Class B Network ID
Required Number of Subnets
Number of Subnet Bits
Subnet Mask
Number of Hosts per Subnet
1-2
1
255.255.128.0 or /17
32,766
3-4
2
255.255.192.0 or /18
16,382
5-8
3
255.255.224.0 or /19
8,190
9-16
4
255.255.240.0 or /20
4,094
17-32
5
255.255.248.0 or /21
2,046
33-64
6
255.255.252.0 or /22
1,022
65-128
7
255.255.254.0 or /23
510
129-256
8
255.255.255.0 or /24
254
257-512
9
255.255.255.128 or /25
126
513-1,024
10
255.255.255.192 or /26
62
1,025-2,048
11
255.255.255.224 or /27
30
2,049-4,096
12
255.255.255.240 or /28
14
4,097-8,192
13
255.255.255.248 or /29
6
8,193-16,384
14
255.255.255.252 or /30
2
Table 1.8 shows the subnetting of a class C network ID.
Table 1.8 Subnetting a Class C Network ID
Required Number of Subnets
Number of Subnet Bits
Subnet Mask
Number of Hosts per Subnet
1-2
1
255.255.255.128 or /25
126
3-4
2
255.255.255.192 or /26
62
5-8
3
255.255.255.224 or /27
30
9-16
4
255.255.255.240 or /28
14
17-32
5
255.255.255.248 or /29
6
33-64
6
255.255.255.252 or /30
2
Step 2: Enumerating Subnetted Network IDs
Based on the number of host bits you use for your subnetting, you must list the new subnetted network IDs. There are two main approaches:
Binary—List all possible combinations of the host bits chosen for subnetting and convert each combination to dotted decimal notation.
Decimal—Add a calculated increment value to each successive subnetted network ID and convert to dotted decimal notation.
Either method produces the same result: the enumerated list of subnetted network IDs.
To create the enumerated list of subnetted network IDs using the binary method
Based on n, the number of host bits chosen for subnetting, create a three-column table with 2n entries. The first column is the subnet number (starting with 1), the second column is the binary representation of the subnetted network ID, and the third column is the dotted decimal representation of the subnetted network ID.
For each binary representation, the bits of the network ID being subnetted are fixed to their appropriate values and the remaining host bits are set to all 0's. The host bits chosen for subnetting vary.
In the first table entry, set the subnet bits to all 0's and convert to dotted decimal notation. The original network ID is subnetted with its new subnet mask.
In the next table entry, increase the value within the subnet bits.
Convert the binary result to dotted decimal notation.
Repeat steps 3 and 4 until the table is complete.
For example, create a 3-bit subnet of the private network ID 192.168.0.0. The subnet mask for the new subnetted network IDs is 255.255.224.0 or /19. Based on n = 3, construct a table with 8 (= 23) entries. The entry for subnet 1 is the all 0's subnet. Additional entries in the table are successive increments of the subnet bits, as shown in Table 1.9. The host bits used for subnetting are underlined.
Table 1.9 Binary Subnetting Technique for Network ID 192.168.0.0
Subnet
Binary Representation
Subnetted Network ID
1
11000000.10101000.00000000.00000000
192.168.0.0/19
2
11000000.10101000.00100000.00000000
192.168.32.0/19
3
11000000.10101000.01000000.00000000
192.168.64.0/19
4
11000000.10101000.01100000.00000000
192.168.96.0/19
5
11000000.10101000.10000000.00000000
192.168.128.0/19
6
11000000.10101000.10100000.00000000
192.168.160.0/19
7
11000000.10101000.11000000.00000000
192.168.192.0/19
8
11000000.10101000.11100000.00000000
192.168.224.0/19
Step 3: Enumerating IP Addresses for Each Subnetted Network ID
Based on the enumeration of the subnetted network IDs, you must now list the valid IP addresses for new subnetted network IDs. To list each IP address individually would be too tedious. Instead, enumerate the IP addresses for each subnetted network ID by defining the range of IP addresses (the first and the last) for each subnetted network ID. There are two main approaches:
Binary—Write down the first and last IP address for each subnetted network ID and convert to dotted decimal notation.
Decimal—Add values incrementally, corresponding to the first and last IP addresses for each subnetted network ID and convert to dotted decimal notation.
Either method produces the same result: the range of IP addresses for each subnetted network ID.
To create the range of IP addresses using the binary method
Based on n, the number of host bits chosen for subnetting, create a three-column table with 2n entries. The first column is the subnet number (starting with 1), the second column is the binary representation of the first and last IP address for the subnetted network ID, and the third column is the dotted decimal representation of the first and last IP address of the subnetted network ID. Alternately, add two columns to the previous table used for enumerating the subnetted network IDs.
For each binary representation, the first IP address is the address in which all the host bits are set to 0 except for the last host bit. The last IP address is the address in which all the host bits are set to 1 except for the last host bit.
Convert the binary representation to dotted decimal notation.
Repeat steps 2 and 3 until the table is complete.
For example, the range of IP addresses for the 3 bit subnetting of 192.168.0.0 is shown in Table 1.11. The bits used for subnetting are underlined.
Table 1.11 Binary Enumeration of IP Addresses
Subnet
Binary Representation
Range of IP Addresses
1
11000000.10101000.00000000.00000001 -11000000.10101000.00011111.11111110
192.168.0.1 - 192.168.31.254
2
11000000.10101000.00100000.00000001 -11000000.10101000.00111111.11111110
192.168.32.1 - 192.168.63.254
3
11000000.10101000.01000000.00000001 - 11000000.10101000.01011111.11111110
192.168.64.1 - 192.168.95.254
4
11000000.10101000.01100000.00000001 -11000000.10101000.01111111.11111110
192.168.96.1 - 192.168.127.254
5
11000000.10101000.10000000.00000001 - 11000000.10101000.10011111.11111110
192.168.128.1 - 192.168.159.254
6
11000000.10101000.10100000.00000001 - 11000000.10101000.10111111.11111110
192.168.160.1 - 192.168.191.254
7
11000000.10101000.11000000.00000001 -11000000.10101000.11011111.11111110
192.168.192.1 - 192.168.223.254
8
11000000.10101000.11100000.00000001 - 11000000.10101000.11111111.11111110
192.168.224.1 - 192.168.255.254
Although the conceptual notion of subnetting by utilizing host bits is straightforward, the actual mechanics of subnetting are a bit more complicated. Subnetting requires a three step procedure:
Determine the number of host bits to be used for the subnetting.
Enumerate the new subnetted network IDs.
Enumerate the IP addresses for each new subnetted network ID.
Step 1: Determining the Number of Host Bits
The number of host bits being used for subnetting determines the possible number of subnets and hosts per subnet. Before you choose the number of host bits, you should have a good idea of the number of subnets and hosts you will have in the future. Using more bits for the subnet mask than required saves you the time of reassigning IP addresses in the future.
The more host bits that are used, the more subnets (subnetted network IDs) you can have — but with fewer hosts. Using too many host bits allows for growth in the number of subnets but limits the growth in the number of hosts. Using too few hosts allows for growth in the number of hosts but limits the growth in the number of subnets.
In practice, network administrators define a maximum number of nodes they want on a single network. Recall that all nodes on a single network share all the same broadcast traffic; they reside in the same broadcast domain. Therefore, growth in the number of subnets is favored over growth in the number of hosts per subnet.
Follow these guidelines to determine the number of host bits to use for subnetting.
Determine how many subnets you need now and will need in the future. Each physical network is a subnet. WAN connections can also count as subnets depending on whether your routers support unnumbered connections.
Use additional bits for the subnet mask if:
You will never require as many hosts per subnet as allowed by the remaining bits.
The number of subnets will increase in the future, requiring additional host bits.
To determine the desired subnetting scheme, start with an existing network ID to be subnetted. The network ID to be subnetted can be a class-based network ID, a subnetted network ID, or a supernet. The existing network ID contains a series of network ID bits that are fixed and a series of host ID bits that are variable. Based on your requirements for the number of subnets and the number of hosts per subnet, choose a specific number of host bits to be used for the subnetting.
Table 1.6 shows the subnetting of a class A network ID. Based on a required number of subnets, and a maximum number of hosts per subnet, a subnetting scheme can be chosen.
Table 1.6 Subnetting a Class A Network ID
Required Number of Subnets
Number of Subnet Bits
Subnet Mask
Number of Hosts per Subnet
1-2
1
255.128.0.0 or /9
8,388,606
3-4
2
255.192.0.0 or /10
4,194,302
5-8
3
255.224.0.0 or /11
2,097,150
9-16
4
255.240.0.0 or /12
1,048,574
17-32
5
255.248.0.0 or /13
524,286
33-64
6
255.252.0.0 or /14
262,142
65-128
7
255.254.0.0 or /15
131,070
129-256
8
255.255.0.0 or /16
65,534
257-512
9
255.255.128.0 or /17
32,766
513-1,024
10
255.255.192.0 or /18
16,382
1,025-2,048
11
255.255.224.0 or /19
8,190
2,049-4,096
12
255.255.240.0 or /20
4,094
4,097-8,192
13
255.255.248.0 or /21
2,046
8,193-16,384
14
255.255.252.0 or /22
1,022
16,385-32,768
15
255.255.254.0 or /23
510
32,769-65,536
16
255.255.255.0 or /24
254
65,537-131,072
17
255.255.255.128 or /25
126
131,073-262,144
18
255.255.255.192 or /26
62
262,145-524,288
19
255.255.255.224 or /27
30
524,289-1,048,576
20
255.255.255.240 or /28
14
1,048,577-2,097,152
21
255.255.255.248 or /29
6
2,097,153-4,194,304
22
255.255.255.252 or /30
2
Table 1.7 shows the subnetting of a class B network ID.
Table 1.7 Subnetting a Class B Network ID
Required Number of Subnets
Number of Subnet Bits
Subnet Mask
Number of Hosts per Subnet
1-2
1
255.255.128.0 or /17
32,766
3-4
2
255.255.192.0 or /18
16,382
5-8
3
255.255.224.0 or /19
8,190
9-16
4
255.255.240.0 or /20
4,094
17-32
5
255.255.248.0 or /21
2,046
33-64
6
255.255.252.0 or /22
1,022
65-128
7
255.255.254.0 or /23
510
129-256
8
255.255.255.0 or /24
254
257-512
9
255.255.255.128 or /25
126
513-1,024
10
255.255.255.192 or /26
62
1,025-2,048
11
255.255.255.224 or /27
30
2,049-4,096
12
255.255.255.240 or /28
14
4,097-8,192
13
255.255.255.248 or /29
6
8,193-16,384
14
255.255.255.252 or /30
2
Table 1.8 shows the subnetting of a class C network ID.
Table 1.8 Subnetting a Class C Network ID
Required Number of Subnets
Number of Subnet Bits
Subnet Mask
Number of Hosts per Subnet
1-2
1
255.255.255.128 or /25
126
3-4
2
255.255.255.192 or /26
62
5-8
3
255.255.255.224 or /27
30
9-16
4
255.255.255.240 or /28
14
17-32
5
255.255.255.248 or /29
6
33-64
6
255.255.255.252 or /30
2
Step 2: Enumerating Subnetted Network IDs
Based on the number of host bits you use for your subnetting, you must list the new subnetted network IDs. There are two main approaches:
Binary—List all possible combinations of the host bits chosen for subnetting and convert each combination to dotted decimal notation.
Decimal—Add a calculated increment value to each successive subnetted network ID and convert to dotted decimal notation.
Either method produces the same result: the enumerated list of subnetted network IDs.
To create the enumerated list of subnetted network IDs using the binary method
Based on n, the number of host bits chosen for subnetting, create a three-column table with 2n entries. The first column is the subnet number (starting with 1), the second column is the binary representation of the subnetted network ID, and the third column is the dotted decimal representation of the subnetted network ID.
For each binary representation, the bits of the network ID being subnetted are fixed to their appropriate values and the remaining host bits are set to all 0's. The host bits chosen for subnetting vary.
In the first table entry, set the subnet bits to all 0's and convert to dotted decimal notation. The original network ID is subnetted with its new subnet mask.
In the next table entry, increase the value within the subnet bits.
Convert the binary result to dotted decimal notation.
Repeat steps 3 and 4 until the table is complete.
For example, create a 3-bit subnet of the private network ID 192.168.0.0. The subnet mask for the new subnetted network IDs is 255.255.224.0 or /19. Based on n = 3, construct a table with 8 (= 23) entries. The entry for subnet 1 is the all 0's subnet. Additional entries in the table are successive increments of the subnet bits, as shown in Table 1.9. The host bits used for subnetting are underlined.
Table 1.9 Binary Subnetting Technique for Network ID 192.168.0.0
Subnet
Binary Representation
Subnetted Network ID
1
11000000.10101000.00000000.00000000
192.168.0.0/19
2
11000000.10101000.00100000.00000000
192.168.32.0/19
3
11000000.10101000.01000000.00000000
192.168.64.0/19
4
11000000.10101000.01100000.00000000
192.168.96.0/19
5
11000000.10101000.10000000.00000000
192.168.128.0/19
6
11000000.10101000.10100000.00000000
192.168.160.0/19
7
11000000.10101000.11000000.00000000
192.168.192.0/19
8
11000000.10101000.11100000.00000000
192.168.224.0/19
Step 3: Enumerating IP Addresses for Each Subnetted Network ID
Based on the enumeration of the subnetted network IDs, you must now list the valid IP addresses for new subnetted network IDs. To list each IP address individually would be too tedious. Instead, enumerate the IP addresses for each subnetted network ID by defining the range of IP addresses (the first and the last) for each subnetted network ID. There are two main approaches:
Binary—Write down the first and last IP address for each subnetted network ID and convert to dotted decimal notation.
Decimal—Add values incrementally, corresponding to the first and last IP addresses for each subnetted network ID and convert to dotted decimal notation.
Either method produces the same result: the range of IP addresses for each subnetted network ID.
To create the range of IP addresses using the binary method
Based on n, the number of host bits chosen for subnetting, create a three-column table with 2n entries. The first column is the subnet number (starting with 1), the second column is the binary representation of the first and last IP address for the subnetted network ID, and the third column is the dotted decimal representation of the first and last IP address of the subnetted network ID. Alternately, add two columns to the previous table used for enumerating the subnetted network IDs.
For each binary representation, the first IP address is the address in which all the host bits are set to 0 except for the last host bit. The last IP address is the address in which all the host bits are set to 1 except for the last host bit.
Convert the binary representation to dotted decimal notation.
Repeat steps 2 and 3 until the table is complete.
For example, the range of IP addresses for the 3 bit subnetting of 192.168.0.0 is shown in Table 1.11. The bits used for subnetting are underlined.
Table 1.11 Binary Enumeration of IP Addresses
Subnet
Binary Representation
Range of IP Addresses
1
11000000.10101000.00000000.00000001 -11000000.10101000.00011111.11111110
192.168.0.1 - 192.168.31.254
2
11000000.10101000.00100000.00000001 -11000000.10101000.00111111.11111110
192.168.32.1 - 192.168.63.254
3
11000000.10101000.01000000.00000001 - 11000000.10101000.01011111.11111110
192.168.64.1 - 192.168.95.254
4
11000000.10101000.01100000.00000001 -11000000.10101000.01111111.11111110
192.168.96.1 - 192.168.127.254
5
11000000.10101000.10000000.00000001 - 11000000.10101000.10011111.11111110
192.168.128.1 - 192.168.159.254
6
11000000.10101000.10100000.00000001 - 11000000.10101000.10111111.11111110
192.168.160.1 - 192.168.191.254
7
11000000.10101000.11000000.00000001 -11000000.10101000.11011111.11111110
192.168.192.1 - 192.168.223.254
8
11000000.10101000.11100000.00000001 - 11000000.10101000.11111111.11111110
192.168.224.1 - 192.168.255.254
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