1. IP

2. Internet Protocol (IP)
Universal service in a heterogeneous world
IP over everything
Globally unique logical address for a host

3. IP Addressing A 32-bit number that uniquely identifies a location
Written using dotted decimal notation

4. IP Addressing IP address is assigned to each network interface
Routers connect two or more physical networks
Each interface has its own address
Multi-homed host
A host having multiple connections to Internet
Multiple addresses identify the same host
Does not forward packets between its interfaces

5. IP Packet

6. IP “Classful” Addressing Scheme
Three unicast address classes: A, B, and C
One multicast: class D
network
host 10
110
1110
multicast address A B C D
class to to to to
32 bits

7. IP Addressing first 24 bits are network address LAN 223.1.1.1
LAN

8. Classless Inter-Domain Routing
Classful addressing scheme wasteful
IP address space exhaustion
A class B net allocated enough for 65K hosts
Even if only 2K hosts in that network
Solution: CIDR
Eliminate class distinction
No A,B,C
Keep multicast class D

9. Classless Addressing Addresses allocated in blocks
Number of addresses assigned always power of 2, and always on the boundary. That is, if 2048 addresses, it will start with some address with all lower 11 bits being 0.
Network portion of address is of arbitrary length
Address format: a.b.c.d/x
x is number of bits in network portion of address
network
part
host
/23

10. Allocating Addresses
Assume abundant addresses are available starting at
Cambridge university needs 2048 addresses, it is given to Mask
Oxford need 4096 addresses. Because the requirement is that must be on the boundary, it is given to Mask
Edinburg needs 1024 addresses, is given to Mask

11. CIDR A router keeps routing table with entries
IP address, 32-bit mask, outgoing line
When an IP packet arrives, the router checks its routing table to find the longest match.
Match means anding the IP address with the network address mask (1111…10000), and check if the result is the same as the network address.

12. CIDR
Example.
Cambridge /
Edinburgh /
(Available) /
Oxford /
When a packet addressing to arrives, where should it be sent to?
And with all masks, find one that matches the longest.

13. CIDR – Entry aggregation
How does a router in Tallahassee route packet to C,E and O, assuming that he has only two outgoing links?
All to New York.
Can it reduce the size of its routing table? C E N O H T

14. CIDR Entry Aggregation
From to , all to N.
So aggregate the three entries into one /19.
The N router can do the same thing. C E N O H T

15. CIDR
If later the free address space / is assigned to Pittsburgh and has to go through Houston, what should the router at Tallahassee do? C E N P O H T

16. CIDR
When a packet arrives addressing , the router checks the routing table and there will be two matches: /22 and /19. Pick the longest match.