Subnetting For Beginners: CIDR, Masks, And /24 Math

An IPv4 address is 32 bits. The “slash” in CIDR notation (the /24 in 192.0.2.0/24) tells you how many of those 32 bits identify the network, and the remaining bits identify the host.

• /24 = 24 network bits + 8 host bits.
• 8 host bits = 2⁸ = 256 total addresses.
• Minus network address and broadcast address = 254 usable hosts.

That is the entire trick. Every subnet question reduces to “how many host bits do I have, and what does 2-to-the-power-of-that look like?”

Memorize these and you can do 90% of day-to-day subnetting in your head:

Prefix   Mask              Total IPs   Usable
/30      255.255.255.252           4         2
/29      255.255.255.248           8         6
/28      255.255.255.240          16        14
/27      255.255.255.224          32        30
/26      255.255.255.192          64        62
/25      255.255.255.128         128       126
/24      255.255.255.0           256       254
/23      255.255.254.0           512       510
/22      255.255.252.0         1,024     1,022
/21      255.255.248.0         2,048     2,046
/20      255.255.240.0         4,096     4,094
/16      255.255.0.0          65,536    65,534
/8       255.0.0.0        16,777,216 16,777,214

Pattern: each step up halves the count, each step down doubles it. Use the subnet calculator for anything that needs to be exact.

For 192.0.2.0/24:

• Network address: 192.0.2.0 — all host bits set to 0.
• First usable host: 192.0.2.1
• Last usable host: 192.0.2.254
• Broadcast address: 192.0.2.255 — all host bits set to 1.

For 192.0.2.16/28 (one of sixteen /28s inside 192.0.2.0/24):

• Network: 192.0.2.16
• First usable: 192.0.2.17
• Last usable: 192.0.2.30
• Broadcast: 192.0.2.31

The pattern is identical at every prefix length — the math just operates on a different number of host bits.

Practical defaults that age well:

• Point-to-point link between two routers: /30 (4 IPs, 2 usable). Or /31 if the gear supports RFC 3021.
• Small office LAN (under 50 devices): /26 (62 usable) gives you headroom.
• Typical office or data-center VLAN: /24 (254 usable). The classic size; familiar to every network engineer.
• Container / pod network: usually /16 or larger, sliced into smaller per-node ranges by the orchestrator.
• Very small reserved block for monitoring, single host on a tunnel: /32.

Pick the smallest subnet that fits two or three years of expected growth. Going too big bloats broadcast domains; going too small forces re-numbering pain later.

IPv6 addresses are 128 bits. The same CIDR notation works: 2001:db8::/32 means 32 network bits and 96 host bits.

The standard subnet size for a LAN in IPv6 is /64. That gives you 2⁶⁴ addresses per subnet — far more than any LAN needs. You do not subnet smaller than /64 for a hosts subnet because SLAAC and several other IPv6 features expect that size.

For a typical end site, the allocation is usually /48 (65,536 LAN-sized /64s) or /56 (256 /64s). Both are deliberately generous — the IPv6 design philosophy is that address scarcity should never drive architecture decisions again.

Once you have picked a CIDR block, two pieces of router config make it work:

• The router interface on that LAN gets an IP inside the block, usually the first or last usable.
• The router announces the block to the rest of the network — statically, via OSPF/BGP, or via your provider's routing protocol of choice.

Inside the block, every host either has a static IP or gets one via DHCP. The router becomes the default gateway; the subnet mask tells each host which destinations are local (same broadcast domain, send by ARP) versus remote (send via gateway).