What does CIDR stand for?
CIDR stands for Classless Inter-domain Routing. The notation that writes a network's prefix length after a slash, such as 192.168.1.0/24, instead of a dotted subnet mask. It allows subnets of any size and lets routers aggregate many networks into one route (supernetting).
CIDR is the way IP networks are written and allocated today: an address followed by a slash and the number of bits that identify the network, such as 192.168.10.0/24. It replaced the old fixed classes A, B and C, so a network can be any size. A /24 holds 256 addresses, of which 254 can be given to hosts; each extra bit in the prefix halves the size. CIDR also lets routers summarize several neighbouring networks in one route. Private ranges such as 10.0.0.0/8 are written the same way and reach the internet through NAT.
CIDR in practice: an example
A department needs addresses for 50 devices. A /24 would waste most of its 254 host addresses. The administrator assigns 192.168.10.64/26, which gives 64 addresses and 62 usable hosts, and keeps the neighbouring /26 blocks free for other departments.
What CIDR is confused with
CIDR vs subnet mask: what is the difference?
They carry the same information in two notations. /24 and 255.255.255.0 both say that the first 24 bits are the network part.
CIDR vs VLSM: what is the difference?
CIDR is the classless notation and allocation scheme. Variable length subnet masking is the practice of using different prefix lengths inside one network, for example a /30 for a link and a /25 for users.
Which exams cover CIDR?
CIDR is on the official CompTIA acronym list for one of the four exams covered here.
- Network+ N10-009: 1.0 (Networking Concepts).
Each number is a CompTIA exam objective and the name in brackets is its domain. The exam puts the term in a scenario; it does not ask you to recite the expansion.
Check yourself: two practice questions
Written by CertSprint for this page. They are not taken from any exam.
1. How many usable host addresses does a /26 network provide?
- 30
- 62
- 64
- 126
Show the answer
B. 62. A /26 leaves 6 bits for hosts, which is 64 addresses. The network and broadcast addresses cannot be assigned, leaving 62.
2. Which subnet mask is the same as the prefix /20?
- 255.255.255.0
- 255.255.240.0
- 255.255.0.0
- 255.240.0.0
Show the answer
B. 255.255.240.0. Twenty bits are 16 bits for the first two octets plus 4 bits in the third, and four leading bits give 240.
Sources: RFC 4632: Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan; RFC 1918: Address Allocation for Private Internets; NIST glossary: Classless Inter-Domain Routing. The acronym and its expansion come from CompTIA's official acronym lists, linked from each exam glossary above. The explanation, example and questions are CertSprint's own, checked against these sources on 11 November 2026.