1. CS 447 Networks and Data Communication ARP (Address Resolution Protocol) for the Internet Department of Computer Science Southern Illinois University Edwardsville Fall, 2015 Dr. Hiroshi Fujinoki E-mail: hfujino@siue.edu IP/000

2. ARP/001 Address Resolution Protocol (ARP) 1. What is ARP? 2. Why ARP is needed? 3. How ARP works? 4. ARP extension to WAN/Internet Presentation Agenda CS 447 Networks and Data Communication

3. ARP/002 Address Resolution Protocol (ARP): 1. It is a mechanism that translates MAC address to IP address MAC (Medium Access Control) address (= hardware address) IP (Internet Protocol) address (= software address) ARP (IP-MAC Translation) e.g. “146.163.147.59” 32-bit address e.g. “1E:32:55:FF:3B:CC” 48-bit address CS 447 Networks and Data Communication

4. ARP/003 What are the software network address and hardware network address? Software Network Address: 1. Network address managed by a network protocol Example:(1) IP address in Internet Protocol (2) “computer name” in Microsoft NetBEUI 2. Network address you can change through OS Hardware Network Address: 1. Network address “hardwired” in each NIC (assigned at a factory) 2. Can not be changed Example: Ethernet MAC address CS 447 Networks and Data Communication

5. ARP/004 Application Presentation Session Transport Network Logical Link Physical IP Address MAC Address NIC Network Host (a terminal computer) The upper 5 layers use IP address The lower two layers use MAC address CS 447 Networks and Data Communication

6. ARP/005 Why ARP is needed? NIC can understand ONLY hardware network address Application Presentation Session Transport Network Logical Link Physical TCP/IP (IP address) (= software address) Ethernet (MAC Address) (= hardware address) ARP CS 447 Networks and Data Communication

7. ARP/007 How ARP works? - Host A wants to send data to host B - Host A knows IP address of B (but not MAC address of B) How A can know MAC address of B? IP = 131.123.45.1 MAC = 1010 A IP = 131.123.45.4 MAC = 1012 B Broadcast “ARP Search” message Sender IP Sender MACReceiver IP Broadcast MAC “ARP Search” LAN CS 447 Networks and Data Communication

8. ARP/008 IP = 131.123.45.1 MAC = 1010 A IP = 131.123.45.4 MAC = 1012 B Broadcast “polling” message Sender IP Sender MACReceiver IP Broadcast MAC “ARP Search” 131.123.45.1 “ARP Search” 1010 131.123.45.40000 Sender IP Sender MACReceiver IPReceiver MAC “ARP Reply” 131.123.45.1 1010 131.123.45.4 1012 “ARP Reply” LAN C CS 447 Networks and Data Communication

11. ARP/009 LAN ARP Host ARP Cache If a host has to broadcast ARP search message for each packet, it is not efficient ARP entries will be cached CS 447 Networks and Data Communication

12. ARP/010 ARP spanning more than one LAN: Now we understand how ARP works for a LAN How ARP can work for a connection spanning more than one LAN? WAN/Internet Router Host A Host B Routers do NOT forward broadcast ARP Search message How host A can know MAC address of B? CS 447 Networks and Data Communication