CIDR and Subnet Masks

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Plain-English explanation

CIDR notation appends the prefix length after a slash: 192.168.10.0/26 means the first 26 bits are the network portion. The subnet mask is the same information in dotted decimal — network bits are 1s, host bits are 0s.

Examples in the last octet:

/24 → 255.255.255.0
/25 → 255.255.255.128
/26 → 255.255.255.192
/27 → 255.255.255.224
/28 → 255.255.255.240

Additional topic reference

IP Classes — Panagiss CCNAmd notes

Deep dive

CIDR Conversion Table — CIDR prefix to subnet mask conversion table.
From study charts · jdepew88 CCNA notes

Binary view of /26 last octet: 11000000 = 128+64 = 192

Contiguous 1s from the left is required for valid subnet masks. 255.255.255.200 is invalid (non-contiguous).

Cross-octet prefixes: /20 means 20 network bits:

Mask: 255.255.240.0 (third octet: 11110000 = 240)
Block size 16 in the third octet

/23, /22, etc.: Practice borrowing bits across octet boundaries — exam favorites.

| Prefix | Mask | Block size (typical) | |--------|------|----------------------| | /8 | 255.0.0.0 | 2^24 in last three octets | | /16 | 255.255.0.0 | 256 in third octet | | /24 | 255.255.255.0 | 256 in fourth octet | | /30 | 255.255.255.252 | 4 addresses — WAN links |

Step-by-step — convert /20 to mask and block

20 network bits → four bits in third octet (16–19)
Third octet binary: 11110000 = 240
Mask: 255.255.240.0
Block size in third octet: 256 − 240 = 16
Networks: x.x.0.0, x.x.16.0, x.x.32.0 …

Commands to know

Cisco interface example

interface GigabitEthernet0/0 ip address 192.168.10.1 255.255.255.192 ! interface GigabitEthernet0/1 ip address 10.10.0.1 255.255.240.0

Verify mask on interface

show ip interface brief show running-config interface GigabitEthernet0/0

Troubleshooting

| Issue | Check | |-------|-------| | Hosts same IP range, can't communicate | Mismatch mask — one /24, one /25 | | Routing summary wrong | Mask doesn't align with summary boundary | | ACL doesn't match intended subnet | Confused mask with wildcard |

Always write CIDR and mask together during design — they are two views of one truth.

Exam relevance

Exam trap

Do not confuse subnet mask with wildcard mask. They are inverses, not the same thing. Subnet 1 = network; wildcard 0 = must match.

60-second method

For last-octet prefixes, list 128, 192, 224, 240, 248, 252 paired with /25–/30 — speed beats brute-force binary on exam day.

Practice checklist

Convert /17, /20, /22, /25, /28 between CIDR and dotted decimal
Spot invalid masks (non-contiguous 1s) in a list of five
Given a mask, state the CIDR prefix without counting bit-by-bit slowly
Configure two interfaces with different masks and verify with show ip int brief
Complete subnetting trainer drills focused on mask conversion

What is the subnet mask for /27?

Which subnet mask is invalid?

Learning objectives