IPv4 addressing, subnetting, and CIDR · Linux

IPv4 is still the dominant layer-3 protocol in production. Its address space is small (about 4.3 billion) and nearly exhausted, but every sysadmin must understand its structure to configure hosts, design subnets, and read firewall rules.

Address format

An IPv4 address is 32 bits, written as four decimal octets:

10.0.0.5
192.168.1.100
203.0.113.42

The leading bits determine the network portion; the trailing bits identify the host. A prefix length (CIDR notation) tells you where the split is.

Prefix	Meaning	Hosts
/32	Single host	1
/30	Point-to-point link	2 usable
/29	Tiny subnet	6 usable
/28	Small subnet	14 usable
/24	Classic class C	254 usable
/16	Class B	65,534 usable
/8	Class A	16,777,214 usable

Two addresses per subnet are always reserved: the network address (all host bits zero) and the broadcast address (all host bits one).

Calculate a subnet

ipcalc 10.10.0.0/24
ipcalc 10.10.0.0/26
ipcalc 10.10.0.50/26 -n

ipcalc is part of the ipcalc package on Debian-derived distros and is in initscripts on RHEL. The -n flag shows the network range.

Worked example: 10.10.0.0/26

The /26 means 26 bits of network and 6 bits of host. 2^6 = 64 addresses total, 62 usable.

Item	Value
Network	10.10.0.0
First usable	10.10.0.1
Last usable	10.10.0.62
Broadcast	10.10.0.63
Total addresses	64
Usable hosts	62

Always leave room for the gateway, monitoring agents, and future growth. A /30 is correct for a point-to-point link but not for a server subnet.

Private address ranges (RFC 1918)

Three ranges are reserved for private use and not routable on the public internet:

Range	Prefix	Use
10.0.0.0 - 10.255.255.255	10.0.0.0/8	Large private networks
172.16.0.0 - 172.31.255.255	172.16.0.0/12	Medium private networks
192.168.0.0 - 192.168.255.255	192.168.0.0/16	Home and small office

There are also the CGNAT range (100.64.0.0/10) and the link-local range (169.254.0.0/16) used by IPv4 zeroconf.

Subnet design discipline

When you design a subnet:

Estimate the host count including monitoring agents, load balancers, and gateway IPs.
Round up to the nearest CIDR that fits with room to spare. A 50-host subnet should be a /25 (126 usable), not a /26.
Document the allocation in an IPAM tool or spreadsheet. Undocumented networks become unmanageable within a year.
Reserve the first 10 and last 10 addresses for network infrastructure (gateway, monitoring, future expansion).

ip -4 addr show
ip -4 route
ip -4 neigh show

The -4 flag forces IPv4 output. Without it, dual-stack hosts show both IPv4 and IPv6, which is usually what you want but sometimes gets in the way of a quick check.