复旦大学：《计算机网络 Computer Networking》课程电子教案（PPT课件讲稿）09 IP Packets

Computer Networking Lecture 9-IP Packets

Computer Networking Lecture 9 – IP Packets

Overview · Last lecture How does choice of address impact network architecture and scalability? What do p 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

9-26-06 Lecture 9: IP Packets 2 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?

P Address classes (Some are Obsolete) Network ID HostID 8 16 24 32 Class a o Network ID Host ID Class B 10 Class C 110 Class D 1110 Multicast Addresses Class E1111 Reserved for experiments 9-26-06 Lecture 9: IP Packets 3

9-26-06 Lecture 9: IP Packets 3 IP Address Classes (Some are Obsolete) Network ID Host ID Network ID Host ID 8 16 Class A 32 0 Class B 10 Class C 110 Class D 1110 Multicast Addresses Class E 1111 Reserved for experiments 24

Outline CIDR IP addressing Forwarding examples ·| P Packet format 9-26-06 Lecture 9: IP Packets

9-26-06 Lecture 9: IP Packets 4 Outline • CIDR IP addressing • Forwarding examples • IP Packet Format

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

9-26-06 Lecture 9: IP Packets 5 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

IP Address Utilization( 97) 一 D http://www.caida.org/outreach/resources/learn/ipv4space/ 9-26-06 Lecture 9: IP Packets 6

9-26-06 Lecture 9: IP Packets 6 IP Address Utilization (‘97) http://www.caida.org/outreach/resources/learn/ipv4space/

Classless Inter-Domain Routing (CIDR)-RFC1338 Allows arbitrary split between network& host part of address Do not use classes to determine network d 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

9-26-06 Lecture 9: IP Packets 7 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

CIDR EXample Network is allocated 8 class c chunks 200.10.0.0to200.10.7.255 Allocation uses 3 bits of class C space Remaining 20 bits are network number, written as201.10.0.0/21 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

9-26-06 Lecture 9: IP Packets 8 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

IP Addresses: How to get one? Network(network portion) Get allocated portion of isPs address space ISPs block 11001000000101110001000000000000200.23.16.0/20 日 Organization0110010000010111000100000020.23160/23 Organization1110010000001011100010010000000200.23180/23 Organization211001000000101110001010000000002002320.0/23 Organization711001000000101110001111000000000200.2330.023 9-26-06 Lecture 9: IP Packets 9

9-26-06 Lecture 9: IP Packets 9 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

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| P address:“ DHCP request”msg DHCP server sends address:“ DHCP ack”msg 9-26-06 Lecture 9: IP Packets 10

9-26-06 Lecture 9: IP Packets 10 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