CIDR Explained: Slash Notation and Route Summarisation
CIDR (Classless Inter-Domain Routing) replaced the old rigid A/B/C classes with flexible slash notation — the /24 in 192.168.1.0/24 simply means "the first 24 bits are the network". This lets networks be any size and, crucially, lets many routes be summarised into one.
What the slash number means
The number after the slash is how many bits, from the left, belong to the network. /24 = 24 network bits = a 255.255.255.0 mask = 256 addresses. /25 = 128 addresses, /23 = 512 addresses. Bigger slash = smaller network. This replaced the wasteful class system where every allocation was forced into a fixed size.
CIDR enables route summarisation
Four consecutive /24 networks (10.1.0.0, 10.1.1.0, 10.1.2.0, 10.1.3.0) can be advertised as a single 10.1.0.0/22 route. Fewer routes mean smaller routing tables and faster convergence — this is why the modern internet, with hundreds of thousands of networks, remains routable. It's a core CCNP skill, built on the CCNA subnetting foundation.
Frequently asked questions
What does /24 mean in an IP address?
/24 means the first 24 bits are the network portion, equivalent to a 255.255.255.0 subnet mask and 256 total addresses (254 usable).
What is the difference between CIDR and classful addressing?
Classful addressing forced networks into fixed A/B/C sizes; CIDR uses slash notation to allow any network size and enables route summarisation, using address space far more efficiently.
What is route summarisation?
Combining several contiguous networks into one larger prefix (e.g. four /24s into a /22) so routers advertise fewer routes — reducing table size and improving convergence.
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