TCPIP Subnetting

The core components of TCP/IP are the IP address and the Subnet mask. The entire purpose of the Subnet Mask is to show the computer where to separate the IP Address into the Network ID and the Host ID. When read in Binary, a Subnet Mask will always be a string of 1's followed by a string of 0's. The 1's cover the Network ID while the 0's cover the Host ID.

The IP Address is separated between the last 1 and the first 0. Network ID Host ID 10101010.01010101 11001100.11100011 = IP Address 11111111.

11111111 00000000.00000000 = Subnet Mask TCP/IP addresses are made up of 4 sets of numbers called "Octets." Each octet is an 8 bit binary string. The largest possible value that can be created with 8 characters in binary is 255. There are three classes of IP Address by default. They are determined by the value of their first octet.

Each Class address has a particular default Subnet Mask. Class "A" = 1-127 Default Subnet Mask = 255.0.

0.0 Class "B" = 128-191 Default Subnet Mask = 255.255.0.

0 Class "C" = 192-223 Default Subnet Mask = 255.255.255.0 When viewed in Binary, you'll notice a pattern. Class "A" always begins with a 0 00000001-01111111 Class "B" always begins with a 10 10000000-10111111 Class "C" always begins with a 110 11000000-11011111 The remaining classes (any with a value of 224 or higher in the first octet) are not used for public use, and are not considered valid. Also, though technically a Class "A" address, 127.

0.0.1 is reserved for testing purposes. It is referred to as the "Loop-Back Address." It, along with any other address beginning with 127 in the first octet, is not considered valid.

When determining Host ID's or Network ID's it is very important to remember that we cannot have an ID containing all 1's or all 0's. All 0's would be an address of 0 (nothing) and all 1's is used to broadcast to everyone within the same group. The easiest and fastest way to determine the number of valid Hosts, Networks, or Subnets, is to count the number of bits (single space digits in Binary) in that ID, figure out the largest possible number (remembering not to use all 1's) and translate that back to decimal. For example: an address with 5 bits dedicated to the Host ID (11110) would have 30 different possible Hosts. This method works to calculate the number of Networks or Subnets as well.

The reverse of this can also be used to calculate how many bits are required for a particular number of Hosts or Subnets. For example: if we had a network that required 50 Hosts, you would determine the number of bits necessary to accommodate the required Hosts. The Binary number of 50 is 110010.

6 bits are required, which means that in reality we would have 62 Hosts (111110 = 62), however if we had tried to use only 5 bits instead of 6 we would end up with 30 Hosts per Subnet. (11110 = 30) Subnetting is used to break a class network into a number of smaller groups of Hosts called a Subnet. For example: a Class "B" network by default has 65,534 Hosts (16 spaces 1111111111111110.) we can break a standard Class "B" network into 30 Subnets each containing 2046 Hosts. 207.119.

87.43 = IP Address 255.255.248.0 = Subnet Mask 11111111.

11111111 11111 000.00000000 Network Subnet Host 5 spaces (11110) = 30 Subnets 11 spaces (11111111110) = 2046 Hosts The Network ID is covered by the default Subnet Mask for the Class address. The Subnet ID is covered by the remaining 1's in the Subnet Mask. The Host ID is covered by the 0's in the Subnet Mask. A network can be subnetted across more then one octet.

We could take a Class "A" address and use it to create a group of Subnets each containing 1022 hosts. 11111111 11111111.111111 00.00000000 Network Subnet Host 112.

107.141.176 = Class "A" address 255.

255.252.0 = Subnet Mask after Subnetting into the third octet. Make sure you verify the address class. Even though the Subnet Mask extends into the third octet, the address is Class "A." In this case, there are 16,382 Subnets each containing 1022 Hosts.

Subnet ID has 14 spaces (11111111111110 = 16382) Host ID has 10 spaces (111111111 = 1022) We can verify the validity of an address simply by converting both the IP Address and the Subnet Mask into Binary, separating the IP address into Network ID, Subnet ID, and Host ID. If any of the IDs is either all 1's or all 0's then the address is not valid. 01101101 0000 1111.10101010 = Not Valid All 0's in the Subnet ID This address would not be valid. To make this address valid we would have to place a valid number in the Subnet ID part of the IP Address, or adjust the Subnet Mask to cover one more bit. 11111111.

11110000. 00000000.00000000 Switch this 0 to a 1.

To determine the range of the IP address, separate the host id from the Network and Subnet IDs. IP Address 10110101.01101001.001 00000.

00000001 Subnet Mask 11111111.11111111.111 00000.00000000 Network/Subnet IDs Host ID Without changing the Network / Subnet Ids, figure the smallest and largest host numbers available. (All 0's with a 1 on the end = Smallest. All 1's with an 0 on the end = Largest.

) Then translate both complete addresses back to decimal. This is your range. 10110101.01101001.001 00000.

00000001 = 181.105.32.1 10110101.

01101001.001 11111.11111110 = 181.105.

63.254 To verify if two addresses are on the same Subnet just translate the IP Addresses and the Subnet into Binary and verify that the Network IDs and Subnet IDs are the same on each IP address. 181.105.112.17 = IP Address #1 10110101.

01101001.011 10000.00010001 181.105.120.129 = IP Address #2 10110101.

01101001.011 11000.11000001 255.255.224.0 = Subnet Mask 11111111.

11111111.111 00000.00000000 The Network IDs must be the same. The Host IDs will be different. Using these steps, we can Ø Identify IP Addresses by Class Ø Specify default Subnet Masks by Class Ø Separate IP Addresses into Network IDs and Host IDs Ø Separate the default Class Addresses into multiple Subnets determined by the required number of Hosts or Subnets Ø Determine the number of Hosts or Subnets in a Network Ø Verify the validity of an address Ø Find the range of a particular Subnet ID Ø Determine whether two addresses are on the same Subnet.

Viko Johns is an ip address technology specialist at ipillion.com. The company offers free ip tracer and ip to location services.