Subnet Mask Cheat Sheet

IT professionals (developers, network engineers) need easy access to subnet mask values and their meanings, so OpenVPN prepared this easy-to-use cheat sheet.

The subnet mask (aka prefix) sets the address range of usable IP addresses (octets made of host and network bits) within a network or subnet. Subnetting breaks out a smaller network from the main network. The 32-bit combination distinguishes two parts of an address: 1) one that refers to the subnet and 2) one that refers to the host. Numbers and decimals make up a subnet mask.

Additional information on networking concepts is available here and includes:

• IPv4 addresses, IPv6 addresses, and private networks.
• Cheat sheet for CIDR notation (the number of bits available to the IP address).

Subnet masks

See also RFC 1878.

Guide to sub-class C blocks

Note: In 1993, Classless Inter-Domain Routing replaced classful IP addressing. CIDR uses five address classes based on the four lead address bits.

• Class A, B, and C: Provide unicast addresses for three network sizes.
• Class D: For multicast networking.
• Class E: Reserved for future and/or experimental use.

/25 — 2 Subnets — 126 Hosts/Subnet

/26 — 4 Subnets — 62 Hosts/Subnet

/27 — 8 Subnets — 30 Hosts/Subnet

/28 — 16 Subnets — 14 Hosts/Subnet

/29 — 32 Subnets — 6 Hosts/Subnet

/30 — 64 Subnets — 2 Hosts/Subnet

Taken from: https://www.aelius.com/njh/subnet_sheet.html

Good to know

• The loopback network in an IPv4 subnet is 127.0.0.0 with a subnet mask of 255.0.0.0.
• A localhost communication port connects to the originating server so a network connection can “loop back” on itself.
• Class C networks use a default subnet mask of 255.255.255.0 and have 192-223 as their first octet.
• The wildcard mask for a Host is always 0.0.0.0.
• A broadcast address, the highest value in the address format, enables transmission to every node in a local network.
• A Variable Length Subnet Mask (VLSM) uses more than one mask in a network to increase subnet usability.