CSE 123 Lecture Notes - Lecture 11: Address Space, Ipv6 Address, Network Address Translation
DepartmentComputer Science and Engineering
Course CodeCSE 123
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IP Address Problem (1991)
- Address space depletion
- Class C too small for most orgs!
CIDR (Classless Inter-Domain Routing) - 1993
- does NOT create any more addresses! (still 32-bit addresses)
- no longer “constrained” to Class A-C addresses!
- networks described by variable-length prefix & length
- allows “arbitrary” allocation between network and host address
| Network | Host
Prefix Mask = # sig bits representing prefix
E.g. 10.95.1.2 contained within 10.0.0.0/8
- 10.0.0.0 is network and remainder (95.1.2) is host
Pro: Finer-grained allocation; aggregation
Con: More expensive lookup: longest prefix match
- only use the longest name in your forwarding table
- can you have 2 network names of the SAME length? NO
- if they’re the same length, same pattern, then they can’t be different! (No ties)
Longest Matching Prefix
- forwarding table contains many prefix/length tuples
- start from the MSBs, ALL the way to the prefix
E.g. 188.8.131.52/20 and 184.108.40.206/23
0001 | 0000
0001 001 | 0 ← longest matching prefix! (23 bits)
Not a simple table! Requires multiple memory lookups
Route Aggregation - if they get a bunch of networks in the same direction, and all “contiguous”
in the sorted list of names, can be sorted in a more efficient way
- combine adjacent networks in forwarding tables!
0001 000 | 0 //220.127.116.11/23
0001 001 | 0 //18.104.22.168/23
0001 010 | 0 //22.214.171.124/23
- send me ANYTHING w/ addresses beginning 126.96.36.199/20
Most Specific Route - but what if address range is NOT contiguous?
- Send me anything with address beginning 188.8.131.52/16 or 184.108.40.206/23 (special and more
- Move in large blocks, not small ones!
- is there a “default” router which matches everything? YES! (# / 0)
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