IP Subnetting

• IP addresses are made up of 32 bits
  • Broken into 4 octets (1 octet = 8 bits)
• The right most bit contains 2^0
  • The octet max is 255
• Octets are borken down to provide an addressing scheme that can accommodate large and small networks
• There are 5 different classes of networks A to E.
  • D and E are reserved
• Class can be determined from 3 high order bits
• In Class A the first octet is the network portion
  • Octets 2, 3, and 4 are for the network manager to divide into subnets and hosts as they see fit
  • Class A addresses are used for netowrks that have more than 65,536 hosts
• In Class B the first two octets are the network portion
  • Used for networks that have between 256 and 65534 hosts
• In Class C the first three octets are the netowrk portion.
  • Class C is perfect for networks with less than 254 hosts

Network Masks

• Network Mask helps you know which portion of the address identifies the network and which portion of the address identifies the node
• Default Masks
  • Class A: 255.0.0.0
  • Class B: 255.255.0.0
  • Class C: 255.255.255.0
• An IP address on a Class A network that has not been subnetter would have an address/mask pair similar to 8.20.15.1 255.0.0.0
  • 8.20.15.1 = 00001000.00010100.00001111.00000001
  • 255.0.0.0 = 11111111.00000000.00000000.00000000
• Any address bits which have corresponding mask bits set to 1 represent the network ID
  • 8.20.15.1 = 00001000.00010100.00001111.00000001
  • 255.0.0.0 = 11111111.00000000.00000000.00000000 ————————————————-

  • = net id

    host id

• netid = 00001000 = 8
• hostid = 00010100.00001111.00000001 = 20.15.1

Understand Subnetting

• Subnetting allows you to create multiple logical networks that exist within a single Class A, B, or C network.
• Any device, or gateway that connects n networks/subnetworks has n distinct IP addresses, one for each network/subnetwork that it interconnects
• In order to subnet a network, extend the natural mask with some of the bits from the host ID portion of the address in order to create a subnetowrk ID.
• For example, given a Class C netowrk of 204.17.5.0 which has a natural mask of 255.255.255.0
  • 204.17.5.0 - 11001100.00010001.00000101.00000000
  • 255.255.255.224 - 11111111.11111111.11111111.11100000 ————————–|sub|—-
• By extending the mask to be 255.255.255.224 you have taken 3 bits from the original host portion of the address and used them to make subnets
• With these 3 bits it is possible to create 8 subnets.
• Each subnet can have up to 32 host addresses
  • 30 of which can actually be assigned to a device
  • Can be denotated as 255.255.255.224 or as /27
  • Since there are 27 masked bits
• Following this example if 2 routers are used:
  • Each router is connected to 4 subnets
    • One of these subnets is common to both routers
• The more host bits you use for a subnet mask, the more subnets you have availble
• However the more subnets available, the less host addresses available per subnet
  • Class C network of 204.17.5.0 and a mask of 255.255.255.224(/27) allows you to have eight subnets, each with 32 host addresses
  • If a mask of 255.255.255.240(/28) the break down is as follows:
  • 204.17.5.0 - 11001100.00010001.00000101.00000000
  • 255.255.255.240 - 11111111.11111111.11111111.11110000 ————————–| sub|—
  • So only 4 bits are used for both subnets and hosts
    • 16 subnets and 16 host address (14 are usable)

Source