What is CIDR notation?

CIDR (Classless Inter-Domain Routing) notation expresses a network address and its prefix length together, such as 192.168.1.0/24. The number after the slash is the prefix length - how many bits of the address identify the network. /24 means the first 24 bits are the network part, leaving 8 bits for host addresses.

What is the difference between a subnet mask and a wildcard mask?

A subnet mask has 1s in the network portion and 0s in the host portion (e.g. 255.255.255.0 for /24). A wildcard mask is the inverse - 0s in the network portion and 1s in the host portion (e.g. 0.0.0.255). Wildcard masks are used in access control lists (ACLs) in Cisco IOS and similar devices.

What are /31 and /32 subnets?

/32 is a host route - a subnet containing exactly one IP address, used to route traffic to a specific host. /31 is a point-to-point subnet (RFC 3021) with exactly two addresses, both usable, with no network or broadcast address. /31 subnets are common on router-to-router links to conserve address space.

What are private IP address ranges?

RFC 1918 defines three private IPv4 ranges not routable on the public internet: 10.0.0.0/8 (Class A), 172.16.0.0/12 (Class B), and 192.168.0.0/16 (Class C). These are used in internal networks, home routers, VPNs, and cloud VPCs.

IP Subnet Calculator

What is subnetting?

Subnetting is the practice of dividing a larger IP network into smaller sub-networks (subnets). This improves routing efficiency, limits broadcast domains, and helps with security segmentation. CIDR (Classless Inter-Domain Routing) notation, like 192.168.1.0/24, replaced the older class-based system and is the universal standard today.

Understanding CIDR prefix lengths

The prefix length (the number after the slash) specifies how many of the 32 bits in an IPv4 address are used to identify the network. The remaining bits identify individual hosts.

/8: 16 777 216 addresses (Class A range)
/16: 65 536 addresses (Class B range)
/24: 256 addresses, 254 usable (Class C range, most common)
/28: 16 addresses, 14 usable (small office / lab segment)
/30: 4 addresses, 2 usable (common for point-to-point WAN links)
/31: 2 addresses, both usable (RFC 3021 point-to-point)
/32: 1 address (host route)

Private address ranges (RFC 1918)

10.0.0.0/8: Class A private range (16 million+ addresses)
172.16.0.0/12: Class B private range (1 million+ addresses)
192.168.0.0/16: Class C private range (65 536 addresses)

These ranges are reserved for private networks and are not routable on the public internet. NAT (Network Address Translation) allows devices on private networks to communicate with the public internet through a single public IP address.

CIDR notation reference table

CIDR prefix	Subnet mask	Total IPs	Usable hosts
/8	255.0.0.0	16,777,216	16,777,214
/16	255.255.0.0	65,536	65,534
/24	255.255.255.0	256	254
/25	255.255.255.128	128	126
/26	255.255.255.192	64	62
/27	255.255.255.224	32	30
/28	255.255.255.240	16	14
/29	255.255.255.248	8	6
/30	255.255.255.252	4	2
/31	255.255.255.254	2	2 (RFC 3021)
/32	255.255.255.255	1	1 (host route)

IPv6 subnetting

IPv6 uses 128-bit addresses instead of 32-bit, written in hexadecimal groups separated by colons (e.g., 2001:0db8:85a3::1). The subnetting concept is identical to IPv4 CIDR, but the scale is dramatically different:

/48: standard allocation for a site or organization. A /48 contains 65,536 possible /64 subnets - enough to give every room in a large building its own subnet.
/64: the standard LAN segment size. A /64 has 2⁶⁴ ≈ 18,446,744,073,709,551,616 host addresses. SLAAC (Stateless Address Autoconfiguration) requires exactly a /64 prefix.
/128: a single host address (equivalent to IPv4's /32 host route).

With IPv6, address conservation is no longer a concern - the total address space (2¹²⁸ ≈ 3.4 × 10³⁸) is large enough to assign every atom on Earth its own address. The focus shifts to hierarchical routing and aggregation rather than conservation.

VLSM (Variable Length Subnet Masking)

VLSM allows a network to be divided into subnets of different sizes from the same address block, eliminating wasted addresses that occur with fixed-size subnetting.

Example: You have a 192.168.1.0/24 block and need to create three subnets - one for 60 hosts, one for 25 hosts, and one for a point-to-point WAN link.

Allocate the largest subnet first: 60 hosts needs a /26 (62 usable addresses) -> 192.168.1.0/26 (addresses .0–.63)
Next: 25 hosts needs a /27 (30 usable addresses) -> 192.168.1.64/27 (addresses .64–.95)
Point-to-point link: 2 hosts needs a /30 (2 usable addresses) -> 192.168.1.96/30 (addresses .96–.99)
Remaining space (192.168.1.100/24 through .255) is available for future subnets.

Always allocate larger subnets first to keep address blocks contiguous and maximize the available remaining space.