15 441 Computer Networking Lecture 9 IP Packets Overview Last lecture How does choice of address impact network architecture and scalability What do IP addresses look like This lecture Modern IP addresses How to get an IP address What do IP packets look like How do routers work 9 26 06 Lecture 9 IP Packets 2 IP Address Classes Some are Obsolete Network ID Host ID 8 16 Class A 0 Network ID 24 32 Host ID Class B 10 Class C 110 Class D 1110 Multicast Addresses Class E 1111 Reserved for experiments 9 26 06 Lecture 9 IP Packets 3 Outline CIDR IP addressing Forwarding examples IP Packet Format 9 26 06 Lecture 9 IP Packets 4 IP Address Problem 1991 Address space depletion In danger of running out of classes A and B Why Class C too small for most domains Very few class A very careful about giving them out Class B greatest problem Class B sparsely populated But people refuse to give it back Large forwarding tables 2 Million possible class C groups 9 26 06 Lecture 9 IP Packets 5 IP Address Utilization 97 http www caida org outreach resources learn ipv4space 9 26 06 Lecture 9 IP Packets 6 Classless Inter Domain Routing CIDR RFC1338 Allows arbitrary split between network host part of address Do not use classes to determine network ID Use common part of address as network number E g addresses 192 4 16 192 4 31 have the first 20 bits in common Thus we use these 20 bits as the network number 192 4 16 20 Enables more efficient usage of address space and router tables How Use single entry for range in forwarding tables Combined forwarding entries when possible 9 26 06 Lecture 9 IP Packets 7 CIDR Example Network is allocated 8 class C chunks 200 10 0 0 to 200 10 7 255 Allocation uses 3 bits of class C space Remaining 20 bits are network number written as 201 10 0 0 21 Replaces 8 class C routing entries with 1 combined entry Routing protocols carry prefix with destination network address Longest prefix match for forwarding 9 26 06 Lecture 9 IP Packets 8 IP Addresses How to Get One Network network portion Get allocated portion of ISP s address space ISP s block 11001000 00010111 00010000 00000000 200 23 16 0 20 Organization 0 11001000 00010111 00010000 00000000 200 23 16 0 23 Organization 1 11001000 00010111 00010010 00000000 200 23 18 0 23 Organization 2 11001000 00010111 00010100 00000000 200 23 20 0 23 Organization 7 11001000 00010111 00011110 00000000 200 23 30 0 23 9 26 06 Lecture 9 IP Packets 9 IP Addresses How to Get One How does an ISP get block of addresses From Regional Internet Registries RIRs ARIN North America Southern Africa APNIC Asia Pacific RIPE Europe Northern Africa LACNIC South America How about a single host Hard coded by system admin in a file DHCP Dynamic Host Configuration Protocol dynamically get address plug and play Host broadcasts DHCP discover msg DHCP server responds with DHCP offer msg Host requests IP address DHCP request msg DHCP server sends address DHCP ack msg 9 26 06 Lecture 9 IP Packets 10 CIDR Illustration Provider is given 201 10 0 0 21 Provider 201 10 0 0 22 9 26 06 201 10 4 0 24 201 10 5 0 24 Lecture 9 IP Packets 201 10 6 0 23 11 CIDR Implications Longest prefix match 201 10 0 0 21 201 10 6 0 23 Provider 1 201 10 0 0 22 201 10 4 0 24 9 26 06 201 10 5 0 24 Provider 2 201 10 6 0 23 or Provider 2 address Lecture 9 IP Packets 12 Outline CIDR IP addressing Forwarding examples IP Packet Format 9 26 06 Lecture 9 IP Packets 13 Host Routing Table Example Destination 128 2 209 100 128 2 0 0 127 0 0 0 0 0 0 0 Gateway 0 0 0 0 0 0 0 0 0 0 0 0 128 2 254 36 Genmask 255 255 255 255 255 255 0 0 255 0 0 0 0 0 0 0 Iface eth0 eth0 lo eth0 From netstat rn Host 128 2 209 100 when plugged into CS ethernet Dest 128 2 209 100 routing to same machine Dest 128 2 0 0 other hosts on same ethernet Dest 127 0 0 0 special loopback address Dest 0 0 0 0 default route to rest of Internet Main CS router gigrouter net cs cmu edu 128 2 254 36 9 26 06 Lecture 9 IP Packets 14 Routing to the Network Packet to 10 1 1 3 arrives Path is R2 R1 H1 H2 10 1 1 2 10 1 1 4 10 1 1 3 H1 H2 10 1 1 24 10 1 0 1 10 1 1 1 10 1 2 2 10 1 0 2 R1 H3 10 1 0 24 10 1 2 23 Provider 10 1 16 R2 10 1 8 1 10 1 2 1 10 1 16 1 10 1 8 24 H4 10 1 8 4 9 26 06 Lecture 9 IP Packets 15 Routing Within the Subnet Packet to 10 1 1 3 Matches 10 1 0 0 23 10 1 1 2 10 1 1 4 H1 Next Hop Interface 127 0 0 1 127 0 0 1 lo0 Default or 0 0 provider 10 1 16 1 10 1 8 0 24 10 1 8 1 10 1 8 1 10 1 2 0 23 10 1 2 1 10 1 2 1 10 1 0 0 23 10 1 2 2 10 1 2 1 9 26 06 H2 10 1 1 24 Routing table at R2 Destination 10 1 1 3 10 1 0 1 10 1 1 1 10 1 2 2 10 1 0 2 R1 H3 10 1 0 24 10 1 2 23 10 1 16 Lecture 9 IP Packets R2 10 1 8 1 10 1 2 1 10 1 16 1 10 1 8 24 H4 10 1 8 4 16 Routing Within the Subnet Packet to 10 1 1 3 Matches 10 1 1 1 31 10 1 1 2 10 1 1 4 H1 10 1 0 1 10 1 1 1 10 1 2 2 Routing table at R1 Next Hop Interface 127 0 0 1 127 0 0 1 lo0 Default or 0 0 10 1 2 1 10 1 2 2 10 1 0 0 24 10 1 0 1 10 1 0 1 10 1 1 0 24 10 1 1 1 10 1 1 4 10 1 2 0 23 10 1 2 2 10 1 2 2 10 1 1 2 31 10 1 1 2 10 1 1 2 9 26 06 H2 10 1 1 24 Longest prefix match Destination 10 1 1 3 10 1 0 2 R1 H3 10 1 0 24 10 1 2 23 10 1 16 Lecture 9 IP Packets R2 10 1 8 1 10 1 2 1 10 1 16 1 …
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