核心 · Key Idea
In one line: A subnet mask tells a host "these high bits are the network part, the rest are the host part". CIDR writes /N for the first N network bits — forget A/B/C classes in modern networking.
What it is#
In 192.168.1.10/24:
- The leading 24 bits (
192.168.1) are the network part — shared by all hosts on that subnet; - The last 8 bits (
.10) are the host part — distinguishes hosts on that subnet.
192.168.1.10/24 in binary:
11000000.10101000.00000001 . 00001010
└────── network (24 bits) ──┘ └ host (8) ┘
The whole subnet 192.168.1.0/24 has 256 addresses; first and last are reserved (.0 = network address, .255 = broadcast). 254 usable.
Analogy#
打个比方 · Analogy
A postal code like 100000 for Beijing — leading digits identify the region, trailing digits the building. The subnet mask is the ruler that tells you how many leading digits represent the region.
Key concepts#
Subnet maskSubnet mask
Same length as the IP (32 bits); 1s = network bits, 0s = host bits. /24 = 255.255.255.0.
CIDR notationClassless Inter-Domain Routing
Write IP/prefix-length, e.g. 10.0.0.0/8, 172.16.0.0/12.
Network addressNetwork address
All host bits zero; identifies the subnet, not assigned to a host.
Broadcast addressBroadcast address
All host bits one; reaches everyone on the subnet.
Usable hostsUsable hosts
2^host_bits - 2 (subtract network + broadcast).
How it works#
A host applies its subnet mask to decide whether the destination is in the same subnet: yes → ARP directly; no → send to the default gateway.
Practical notes#
- CIDR cheatsheet:
| CIDR | Mask | Usable hosts |
|---|---|---|
| /24 | 255.255.255.0 | 254 |
| /25 | 255.255.255.128 | 126 |
| /26 | 255.255.255.192 | 62 |
| /28 | 255.255.255.240 | 14 |
| /30 | 255.255.255.252 | 2 (point-to-point) |
/32= a single host — common in routing-table specifics./0= "all IPs"; the default route0.0.0.0/0.- Don't compute by hand: use
ipcalc 192.168.1.0/26to print network / broadcast / range. - Smaller subnets save addresses but bloat routing tables. In practice, slicing by
/24per business unit balances manageability over conservation.
Easy confusions#
A / B / C classes
Early fixed split: A=/8, B=/16, C=/24.
Too coarse and wasteful — **deprecated**.
Too coarse and wasteful — **deprecated**.
CIDR
Arbitrary prefix length /N.
The modern internet's scheme.
The modern internet's scheme.