Network & IP Reference

IP Subnet Reference Chart

Quick reference for CIDR prefix sizes, subnet masks, usable host counts, private address ranges, and special addresses. Use for fast planning, then use the calculator for exact results.

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Terms

CIDRClassless Inter-Domain Routing: The notation used to write a block of network addresses and its size in one compact form, like 192.168.1.0/24 — the number after the slash says how many addresses are in the block.
VLANVirtual Local Area Network: A way to split one physical network into several separate, isolated virtual networks using the same cables and switches — like having several private lanes on one road instead of building separate roads.
WANWide Area Network: A network that connects locations across a large distance — like linking a company's different office buildings or connecting to the internet — as opposed to the network inside a single building (LAN).

CIDR prefix quick reference

Usable host count = 2^{(32−prefix)} − 2. The subtracted 2 accounts for the network address and broadcast address, which cannot be assigned to hosts. Exception: /31 and /32 follow different rules (RFC 3021 and host routes).

Prefix	Subnet mask	Usable hosts	Subdivision	Typical use
/8	`255.0.0.0`	16 777 214	1 class A	ISP and large enterprise backbones
/16	`255.255.0.0`	65 534	256 class C	Campus networks, large office segments
/20	`255.255.240.0`	4 094	16 per /16	Mid-size VLANs, cloud VPCs
/21	`255.255.248.0`	2 046	8 per /18	Medium-sized segments
/22	`255.255.252.0`	1 022	4 per /20	Large VLANs, data centre tiers
/23	`255.255.254.0`	510	2 per /22	Department segments
/24	`255.255.255.0`	254	256 per /16	Most common: office VLAN, small site
/25	`255.255.255.128`	126	2 per /24	Split /24 for two segments
/26	`255.255.255.192`	62	4 per /24	Small server VLANs
/27	`255.255.255.224`	30	8 per /24	Management VLANs, small clusters
/28	`255.255.255.240`	14	16 per /24	Point-to-point with room to grow
/29	`255.255.255.248`	6	32 per /24	Router-to-router links or tiny segments
/30	`255.255.255.252`	2	64 per /24	Classic point-to-point WAN links
/31	`255.255.255.254`	2	128 per /24	RFC 3021 P2P — no broadcast overhead
/32	`255.255.255.255`	1 host	Host route	Loopback, host routes, firewall rules

RFC 1918 private address ranges

These three blocks are reserved for private use. They are not routed on the public internet. Use NAT or a VPN tunnel to reach the internet from these ranges.

Range	Block	Size	Common use
10.0.0.0/8	`10.x.x.x`	16 777 216 addresses	Large enterprises, cloud VPCs
172.16.0.0/12	`172.16–31.x.x`	1 048 576 addresses	Mid-size networks, VPN ranges
192.168.0.0/16	`192.168.x.x`	65 536 addresses	Home and small office LANs

Avoid:Using 192.168.x.x for large enterprise networks — the block is too small and will conflict with home routers when employees use VPN. Prefer 10.x.x.x for corporate address plans.

Special-purpose addresses

These ranges are reserved by IANA for specific functions and must not be used as host addresses.

Range	Purpose
`127.0.0.0/8`	Loopback — local host only
`169.254.0.0/16`	Link-local (APIPA) — no DHCP server reached
`224.0.0.0/4`	IPv4 multicast — routing protocols, mDNS
`255.255.255.255`	Limited broadcast — never routed
`0.0.0.0/0`	Default route — catch-all in routing tables

Subnetting workflow

Count the maximum number of hosts needed in the segment.
Add a growth buffer — typically 20–50 % extra address space.
Choose the smallest CIDR prefix that covers hosts + buffer.
Verify no overlap with existing subnets in the address plan.
Reserve .0 (network) and .255 (broadcast) — they are not assignable.
Document the subnet, VLAN ID, gateway, DHCP range, and purpose.
Use the IP Subnet Calculator to verify host counts and ranges before deploying.

Common mistakes

Using /24 for every subnet regardless of actual host count.
Forgetting network address (.0) and broadcast (.255) reduce usable host count by 2.
Overlapping subnets across sites or VLANs causing silent routing failures.
Not leaving address space for planned growth (decommissioning and re-subnetting is expensive).
Mixing RFC1918 ranges without a clear addressing plan.
Using /30 for point-to-point links when /31 (RFC 3021) is supported and wastes no addresses.

Address formula

Key relationships to keep in mind when designing an IP address plan:

Usable hosts = 2^(32 − prefix) − 2

Subnet count from parent = 2^(new_prefix − parent_prefix)

Network address = host address AND subnet mask

Broadcast = network address OR (inverted mask)

Example: 192.168.10.0/25 → network .0, broadcast .127, gateway .1, usable .2 – .126 (126 hosts)

FAQ

Why are there only 254 usable hosts in a /24?

A /24 has 256 total addresses (2⁸). The first (.0) is the network address and the last (.255) is the broadcast address. Both are reserved and cannot be assigned to hosts, leaving 254 usable addresses.

What is the difference between /30 and /31 for point-to-point links?

A /30 provides 4 total addresses (2 usable + network + broadcast). A /31 follows RFC 3021: both addresses are usable with no network or broadcast overhead, saving 2 addresses per link. Most modern routers support /31 — check vendor documentation before deploying.

Can I split a /24 into smaller subnets?

Yes. A /24 can be split into two /25s, four /26s, eight /27s, and so on. Each additional bit in the prefix halves the host count and doubles the subnet count. Use the IP Subnet Calculator to verify ranges and avoid overlap.

Next step

Need exact host ranges?

Use the IP Subnet Calculator to enter a network address and prefix length and get the exact network address, broadcast, first/last host, wildcard mask, and usable host count.

Open subnet calculator →

Disclaimer

This reference chart provides planning guidance only. Always verify subnet ranges, host counts, and address assignments against your authoritative IP address management (IPAM) system before deploying in production environments. Overlapping subnets cause routing failures that can be difficult to diagnose.