Subnet
From Cheatsheet
Contents |
Subnetting and Tables
Subnetting is the concept of dividing the network into smaller portions called subnets. Link to original source: click here. This is done by borrowing bits from the host portion of the IP address, enabling more efficient use of the network address. A subnet mask defines which portion of the address is used to identify the network and which denotes the hosts.
The following tables show all possible ways a major network can be subnetted, and, in each case, how many effective subnets and hosts are possible.
There are three tables, one for each class of addresses.
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The first column shows how many bits are borrowed from the host portion of the address for subnetting.
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The second column shows the resulting subnet mask in dotted decimal format.
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The third column shows how many subnets are possible.
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The fourth column shows how many valid hosts are possible on each of these subnets.
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The fifth column shows the number of subnet mask bits.
Class A Host/Subnet Table
Class A Number of Bits Borrowed Subnet Effective Number of Number of Subnet from Host Portion Mask Subnets Hosts/Subnet Mask Bits ------- --------------- --------- ------------- ------------- 1 255.128.0.0 2 8388606 /9 2 255.192.0.0 4 4194302 /10 3 255.224.0.0 8 2097150 /11 4 255.240.0.0 16 1048574 /12 5 255.248.0.0 32 524286 /13 6 255.252.0.0 64 262142 /14 7 255.254.0.0 128 131070 /15 8 255.255.0.0 256 65534 /16 9 255.255.128.0 512 32766 /17 10 255.255.192.0 1024 16382 /18 11 255.255.224.0 2048 8190 /19 12 255.255.240.0 4096 4094 /20 13 255.255.248.0 8192 2046 /21 14 255.255.252.0 16384 1022 /22 15 255.255.254.0 32768 510 /23 16 255.255.255.0 65536 254 /24 17 255.255.255.128 131072 126 /25 18 255.255.255.192 262144 62 /26 19 255.255.255.224 524288 30 /27 20 255.255.255.240 1048576 14 /28 21 255.255.255.248 2097152 6 /29 22 255.255.255.252 4194304 2 /30 23 255.255.255.254 8388608 2* /31
Class B Host/Subnet Table
Class B Subnet Effective Effective Number of Subnet Bits Mask Subnets Hosts Mask Bits ------- --------------- --------- --------- ------------- 1 255.255.128.0 2 32766 /17 2 255.255.192.0 4 16382 /18 3 255.255.224.0 8 8190 /19 4 255.255.240.0 16 4094 /20 5 255.255.248.0 32 2046 /21 6 255.255.252.0 64 1022 /22 7 255.255.254.0 128 510 /23 8 255.255.255.0 256 254 /24 9 255.255.255.128 512 126 /25 10 255.255.255.192 1024 62 /26 11 255.255.255.224 2048 30 /27 12 255.255.255.240 4096 14 /28 13 255.255.255.248 8192 6 /29 14 255.255.255.252 16384 2 /30 15 255.255.255.254 32768 2* /31
Class C Host/Subnet Table
Class C Subnet Effective Effective Number of Subnet Bits Mask Subnets Hosts Mask Bits ------- --------------- --------- --------- -------------- 1 255.255.255.128 2 126 /25 2 255.255.255.192 4 62 /26 3 255.255.255.224 8 30 /27 4 255.255.255.240 16 14 /28 5 255.255.255.248 32 6 /29 6 255.255.255.252 64 2 /30 7 255.255.255.254 128 2* /31
Subnetting Example
The first entry in the Class A table (/10 subnet mask) borrows two bits (the leftmost bits) from the host portion of the network for subnetting, then with two bits you have four (22) combinations, 00, 01, 10, and 11. Each of these will represent a subnet.
Binary Notation Decimal Notation -------------------------------------------------- ----------------- xxxx xxxx. <b>00</B>00 0000.0000 0000.0000 0000/10 ------> X.0.0.0/10 xxxx xxxx. <b>01</B>00 0000.0000 0000.0000 0000/10 ------> X.64.0.0/10 xxxx xxxx. <b>10</B>00 0000.0000 0000.0000 0000/10 ------> X.128.0.0/10 xxxx xxxx. <b>11</B>00 0000.0000 0000.0000 0000/10 ------> X.192.0.0/10
