IPv4 Address Classes – Class A (Free Preview)

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  • Class A addresses are used to assign to networks with large number of hosts.
  • The whole of first octet is used for the network portion.
  • The class A network IDs have a prefix length of /8 (255.0.0.0.0).
  • The most significant bit of the high-order octet of a class A address is ALWAYS set to 0. The remaining 7 bits in the first octet are available to create Network IDs (Network Addresses). Therefore, the most number of network IDs we can create is 128 (2^7).

 

Figure The structure of class A addresses

n: indicates a bit used for the network ID. h: indicates a bit used for the host ID.

 

 

 

Figure Class A Specifics

 

  • *The class A addresses 127.0.0.1 to 127.255.255.254 (127.0.0.0/8) is reserved for loopback IPv4 addresses.
  • *The address prefix 0.0.0.0 /0 (or 0.0.0.0 0.0.0.0) is a reserved network ID.
  • The maximum number of available class A Network IDs is 128. Since we cannot use 0.0.0.0 /8 and 127.0.0.0 /8, there are only 126 usable Network IDs a network engineer can use.
  • The maximum number of hosts per class A network is 16,777,214 {(2^24 – 2) = (16,777,214 – 2)}.

 

Note: For any host ID,

  1. All bits within the host portion CANNOT be set to 1 because this host ID is reserved as a broadcast address.
  2. Also, all bits in the host portion CANNOT be set to 0 because this signifies the IP network ID.

When calculating the number of usable hosts, we apply the formula 2^n -2. The n is the number of available bits in the host portion and “-2” is to deduct two addresses we cannot use due to above two reasons. 

 

 

Figure Class A summary

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