IP Class Finder

Tells you the class, default mask, address range and any special designation (loopback, APIPA, multicast) for any IPv4 address. Networks stopped actually using class-based addressing decades ago in favour of CIDR, but the classification still shows up constantly in exam questions and in older documentation you'll inherit from someone else.

The classes, for reference

• Class A (1–126.x.x.x) — default /8, 16 million hosts per network. 127.x.x.x is carved out specifically for loopback, not assigned as a usable Class A block.
• Class B (128–191.x.x.x) — default /16, 65,534 hosts per network. Historically the size handed to universities and large enterprises.
• Class C (192–223.x.x.x) — default /24, 254 hosts. The size most people actually deal with day to day.
• Class D (224–239.x.x.x) — reserved for multicast, never assigned to individual hosts.
• Class E (240–255.x.x.x) — reserved, experimental, not seen in production.

Special addresses worth recognising on sight

• 127.0.0.1 — loopback. Never leaves the host, which is exactly what makes it useful for local testing.
• 0.0.0.0 — "this network," and the address that shows up as the default route in a routing table.
• 169.254.x.x — APIPA, assigned automatically when DHCP fails. Seeing this address on a machine is a strong hint the actual problem is DHCP, not whatever symptom brought you to look at the IP in the first place.
• 255.255.255.255 — limited broadcast, sent to every host on the local segment.
• RFC 1918 ranges — 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 — private, non-routable on the public internet, and worth being able to recognise instantly since so much troubleshooting starts with "is this a public or private address."