Subnet Masks

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• relative size of the network and host portions of IPv4 address determines how many networks and hosts per network an addressing scheme can support
  • conventional scheme has IPv4 address with two hierarchical levels:
    • network ID
    • host ID
  • this scheme is inflexible
  • so system of dividing networks in to subnetworks was devised
• subnet addressing has 3 hierarchical levels:
  • network ID
  • subnet ID
  • host ID
• to create logical subnets:
  • bits from host portion of IP must be allocated as subnetwork address
    • rather than part of the host ID
    • means that subnet ID lies within an octet boundary
    • E.g., binary mask with 28 bits could use all the octets, with the network prefix boundary lying in the fourth octet:
      
      • leaves only 4 bits for host ID range
      • mask for the whole network is still 255.255.255.0
      • hosts within the network use the subnet mask 255.255.255.240
  • only one mask is ever applied to the IP address on each interface
  • mask containing the subnet information is only used within the IP network
  • External IP networks address the whole network by its network ID (198.51.100.0/255.255.255.0)
  • hosts within the network use the longer subnet mask to differentiate subnets
    • these are:
      • 198.51.100.0/255.255.255.240
      • 198.51.100.16/255.255.255.240
      • 198.51.100.32/255.255.255.240
      • 198.51.100.48/255.255.255.240
      • etc.
  • because 1s in mask are always contiguous, each octet in decimal in an IPv4 will always be one of these:

Octet Mask Bits	Binary Octet	Decimal Equivalent	Usable Addresses
1	10000000	128	128
2	11000000	192	64
3	11100000	224	32
4	11110000	240	16
5	11111000	248	8
6	11111100	252	4
7	11111110	254	2
8	11111111	255	0

• memorize these values to convert masks between binary and decimal
  • E.g.,
    • mask has 14 bits, then octet is 8 bits + 6 bits
    • so /14 network has the following mask:
    • 11111111 11111100 00000000 00000000 = 255.252.0.0