- 2071 Chaitra · 4+4 marks
Need of classless IP addressing
Short answer
Classful addressing wasted addresses (e.g., 1000 hosts needed Class B wasting 64k), exhausted address space, bloated routing tables, and prevented aggregation. Classless CIDR allows variable prefix lengths; example 200.23.16.0/22 gives 1024 addresses (mask 255.255.252.0) for 1000 hosts, aggregating four Class C networks into one routing entry.
Why do we think that there arose the need of classless IP address although class based IP address was in use? Show the classless IP with an example.
Class-based addressing divides the IPv4 space into fixed blocks (Class A /8, Class B /16, Class C /24). Classless addressing (CIDR, Classless Inter-Domain Routing) removes the fixed class boundaries and lets the network part be any length, written as a.b.c.d/n.
Why classless addressing was needed
- Wasted addresses. An organisation needing 1,000 hosts is too big for a Class C (254 hosts) and received a Class B (65,534 hosts), wasting about 64,000 addresses.
- Address exhaustion. Class A and B blocks were handed out quickly, and the 32-bit space was running out in the early 1990s.
- Huge routing tables. Giving an organisation many separate Class C blocks added one routing entry per block, and backbone routers could not keep up.
- No aggregation. Class-based addresses cannot be summarised, but CIDR blocks can be combined into one prefix (supernetting).
- Fixed sizes. Only three block sizes exist; real needs (500, 1,000, 2,000 hosts) do not fit.
Classless IP with an example
An address block is given as a prefix and mask: the first n bits are the network prefix, the remaining 32 − n bits identify hosts.
Example: an ISP gives a company with 1,000 hosts the block 200.23.16.0/22.
Prefix length : 22 bits -> mask 255.255.252.0
Host bits : 10 -> 2^10 = 1024 addresses
Network address: 200.23.16.0
Usable hosts : 200.23.16.1 - 200.23.19.254
Broadcast : 200.23.19.255
The block is equal to four Class C networks (200.23.16.0, .17.0, .18.0, .19.0). Outside routers keep one entry (200.23.16.0/22) instead of four. Only 22 usable host addresses are unused, instead of about 64,000 with a Class B.
| Point | Classful | Classless (CIDR) |
|---|---|---|
| Block size | /8, /16, /24 only | any /n |
| Mask | implied by first bits | always given with the address |
| Aggregation | not possible | possible |
| Wastage | high | low |
✓ Answer checked by Claude Opus 5.5 (high effort)
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