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Lecture 11

CSE 123 Lecture Notes - Lecture 11: Address Space, Ipv6 Address, Network Address Translation


Department
Computer Science and Engineering
Course Code
CSE 123
Professor
Alex Snoeren
Lecture
11

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IP Address Problem (1991)
- Address space depletion
Why?
- 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. 200.23.16.0/20 and 200.23.18.0/23
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!
E.g.: 200.23.16.0/23
0001 000 | 0 //200.23.16.0/23
0001 001 | 0 //200.23.18.0/23
0001 010 | 0 //200.23.20.0/23
- send me ANYTHING w/ addresses beginning 200.23.16.0/20
Most Specific Route - but what if address range is NOT contiguous?
- Send me anything with address beginning 199.31.0.0/16 or 200.23.18.0/23 (special and more
specific)
- Move in large blocks, not small ones!
- is there a “default” router which matches everything? YES! (# / 0)
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