Transcript ICOM 6115
ICOM 6115 – Computer Networks and the WWW Manuel Rodriguez-Martinez, Ph.D. Lecture 22 ICOM 6115 ©Manuel Rodriguez-Martinez Lecture Objectives • Introduction to Global Internetworking – Layer 3 – The Network Layer • Store-and-Forward Service – IP Protocol • IP addressing • ARP, ICMP • DHCP ICOM 6115 ©Manuel Rodriguez-Martinez IP Addresses • Unique, Global identifier for a network card – Machines with multiple card get multiple IPs • e.g. routers • IP address is a 32 bit number – For simplicity, expressed in dotted-decimal notation • e.g. 128.34.5.92 Bits 32 128 ICOM 6115 34 0 5 ©Manuel Rodriguez-Martinez 92 What are benefits of IP Addresses? • Hierarchical structure – Provide way to identify network and host • Ethernet are flat – Example: 135.134.9.20 • Host 20 • Network 135.134.9.0 • Routers need only to keep information about network – subnets ICOM 6115 ©Manuel Rodriguez-Martinez IP Address Formats ICOM 6115 ©Manuel Rodriguez-Martinez Special (Reserved) IP addresses ICOM 6115 ©Manuel Rodriguez-Martinez Some Facts • Class A addresses – 7 bits for network, 24 bits for host • 126 Class A network, with roughly 2^24 hosts • Class B addresses – 14 bits for network, 16 bits for host • 64K hosts on a Class B network • Class C addresses – 21 bits for network, 8 bits for host • 255 hosts a on a Class C network ICOM 6115 ©Manuel Rodriguez-Martinez Issues: What’s in a forwarding table? • A router cannot deal with 64K IP addresses in a table • Solution: Subnetting – Use the host part to identify areas of the LAN • Subnet – All host on a given subnet share: • Same network part of IP address • Same subnet part of IP address – Routers mainly store IP address of networks and subnets ICOM 6115 ©Manuel Rodriguez-Martinez Example a Campus with Subnets ICOM 6115 ©Manuel Rodriguez-Martinez IP address with subnets • MASK = Bit pattern that can be used to inspect certain bits on a byte, word, or array of byte • Subnet MASK – used to inspect the bits that indicate IP Address of the subnet Subnet mask is often written in dotted decimal notation e.g. 255.255.252.0 Also written in terms of bit length: /22 ICOM 6115 ©Manuel Rodriguez-Martinez Examples • Suppose we have the network address – 130.50.0.0 • 10000010 00110010 00000000 00000000 • Let the subnet mask be: 255.255.252.0 • The structure of the mask is • 11111111 11111111 11111100 00000000 • Subnets • 130.50.4.0 - 11111111 11111111 00000100 00000000 • 130.50.8.0 - 11111111 11111111 00001000 00000000 • 130.50.12.0 - 11111111 11111111 00001100 00000000 ICOM 6115 ©Manuel Rodriguez-Martinez Sample LAN ICOM 6115 ©Manuel Rodriguez-Martinez Forwarding Tables entries ICOM 6115 SubnetNumber SubnetMask NextHop 128.96.34.0 255.255.255.128 Interface 0 128.96.34.128 255.255.255.128 Interface 1 128.96.33.0 255.255.255.0 R2 ©Manuel Rodriguez-Martinez Datagram forwarding Algorithm D = destination IP address For each entry I = (SubnetNumber, SubnetMask,NextHop) in forwarding table T D1 = MASK & D; // bitwise AND if D1 == SubnetNumber if NextHop is an interface deliver datagram directly else deliver to nextHop router ICOM 6115 ©Manuel Rodriguez-Martinez More examples ICOM 6115 ©Manuel Rodriguez-Martinez Issue how to map: IP address to MAC • ARP – Address Resolution Protocol – Sender broadcast a message asking host with IP X to reply with MAC address • Cached for later – Only that machine should reply – Other host will see the message and react • If they have MAC address of X in memory they will refresh it • Otherwise, they ignore it ICOM 6115 ©Manuel Rodriguez-Martinez ARP Packet format ICOM 6115 ©Manuel Rodriguez-Martinez Issue: IP addresses are scarce • To most sites Class A and C are not appropriate – Too many hosts or too little • Every body wants a Class B network address • Problem: Not enough IPs to go around • Solutions – IPv6 – NAT ICOM 6115 ©Manuel Rodriguez-Martinez NAT: Network Address Translation ICOM 6115 ©Manuel Rodriguez-Martinez