What are the permissible IP address ranges?

The permissible IP address ranges depend on the protocol version:

• IPv4: addresses consist of 4 octets (0-255). There are special ranges:

  • Private networks:
    • 10.0.0.0 – 10.255.255.255
    • 172.16.0.0 – 172.31.255.255
    • 192.168.0.0 – 192.168.255.255
  • Reserved addresses, for example 127.0.0.1 for localhost.
  • Public addresses — all others not in the special ranges.

• IPv6: addresses consist of 128 bits, written as 8 groups of 4 hexadecimal digits.

  • Special ranges, for example:
    • ::1 — localhost
    • fc00::/7 — unique local addresses
    • fe80::/10 — link-local addresses

Overall, a valid IP address must conform to the format and not belong to the reserved ranges for special purposes if it is about public addressing.