ICOM 6115 Computer Networks and the WWW Manuel

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 32 128 ICOM 6115 34 Bits 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 • 64 K 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 64 K 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. 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 • Let the subnet mask be: 255. 252. 0 • The structure of the mask is • 1111111100 0000 • Subnets • 130. 50. 4. 0 - 11111111 00000100 0000 • 130. 50. 8. 0 - 11111111 00001000 0000 • 130. 50. 12. 0 - 11111111 00001100 0000 ICOM 6115 ©Manuel Rodriguez-Martinez

Sample LAN ICOM 6115 ©Manuel Rodriguez-Martinez

Forwarding Tables entries ICOM 6115 Subnet. Number Subnet. Mask Next. Hop 128. 96. 34. 0 255. 128 Interface 0 128. 96. 34. 128 255. 128 Interface 1 128. 96. 33. 0 255. 0 R 2 ©Manuel Rodriguez-Martinez

Datagram forwarding Algorithm D = destination IP address For each entry I = (Subnet. Number, Subnet. Mask, Next. Hop) in forwarding table T D 1 = MASK & D; // bitwise AND if D 1 == Subnet. Number if Next. Hop is an interface deliver datagram directly else deliver to next. Hop 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 – IPv 6 – NAT ICOM 6115 ©Manuel Rodriguez-Martinez

NAT: Network Address Translation ICOM 6115 ©Manuel Rodriguez-Martinez