IPv4 addressing and subnet calculations

IPv4 network and host bits

IPv4 uses 32 bits, usually written as four numbers from 0 to 255. A prefix such as /24 means 24 bits identify the network. The remaining eight bits give 256 address values. A conventional /24 LAN has 254 usable host addresses because the network and broadcast values have special jobs.

192.168.10.0/26 contains addresses .0 through .63. The mask is 255.255.255.192; .0 is the network, .63 is the broadcast, and .1 through .62 are usable hosts. The next /26 begins at .64. The gateway is a configured router address in the subnet, not automatically “the first address”.

Subnet calculation procedure

1. Find how many host bits remain: 32 minus the prefix.
2. Find the block size: 2 raised to that number.
3. Locate the containing network boundary.
4. Find the last address in the block, then the usable range.

The usual hosts formula is 2 to the host-bits power minus 2. /31 point-to-point links and /32 host routes are special cases; do not apply that formula blindly. Variable-length subnetting uses different prefix lengths for different needs. For 50 ordinary LAN hosts, /26 gives enough space; /27 gives only 30 usable hosts.

Private IPv4 ranges are 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. Private addressing is not encryption or a firewall.