Transcript Chapter 3
Chapter 3 Address Resolution Protocol (ARP) ARP • ARP is a protocol that is part of the TCP/IP protocol suite • Responsible for translating a logical IP address into a hardware address Hardware Address • Consists of six two-digit hexadecimal characters separated by hyphens • Example: 00-E0-98-00-DE-D5 • Pre-assigned to network interface cards when they are manufactured • Also called: – – – – NIC address MAC address – most common term Physical address Ethernet address Hardware Address – Cont. • Hardware address is unique • Used to identify source and destination • No good in identifying what network the host is located on • Difficult for humans to remember IP addresses • Logical address that specifies what network the host is on as well as identifies a specific host on the network • Used by TCP/IP protocol suite • Example: 118.14.126.57 • 32 bits in length • Some bits specify the network segment • Subnet mask is used to determine which bits specify the network • Example subnet: 255.255.255.0 IP Conversion • When a packet of data is being encapsulated on the source computer, the destination’s hardware address must be included in the packet header. • ARP is responsible for determining the hardware address of the destination before the frame can be sent out ARP • ARP determines the MAC address of the destination from the destination’s IP address • This process is called resolving the address Host Initialization • Each host must initialize itself on the local network • The host sends out an ARP broadcast containing its IP address and its hardware address to all hosts on the local network Local Destination Host Vs. Remote Destination Host • The sending host can determine if the frame is being sent to a local host or a remote host • This can be determined from the destination’s IP address • Example: – – – – Source IP: 118.14.126.17 Destination IP: 118.14.126.32 Subnet Mask: 255.255.255.0 Local destination in this example Resolution on a Local Network • ARP cache on the host sending the frame is checked to see if the destination IP address is found. If so, the specified MAC address is used • If no mapping is found in the local ARP cache, an ARP request is sent out • This request asks: “Who does this IP address belong to and what is your hardware address?” • This ARP request is sent out as a broadcast frame which goes to each host on the hetwork Local ARP Resolution – Cont. • Each host receiving the ARP request checks the IP address to see if it matches theirs • If the IP address does not match, the request is ignored • If the IP address does match, an ARP reply is sent back to the source host that originated the ARP request • The ARP reply can be sent back because the source IP and hardware address are contained in the ARP request Local ARP Resolution – Cont. • The ARP cache on both machines is updated with the correct IP and corresponding hardware addresses • Communication can now be established between the two hosts Remote ARP Resolution • In this case, the destination is located on a remote network instead of the local network • The destination will have to be reached through one or more routers • ARP must resolve the address of each router that stands between the source and destination • Each stop through a router is called a hop • ARP adds the hardware address for the first router when sending the frames out Remote ARP Resolution – Cont. • The IP address of the destination is determined to be remote • The routing table on the source is checked to see if a known route exists to the network that the destination host is on • If a route is found,the ARP cache is checked to see whether the mapping to the router that’s needed to reach the destination is in place • If no route is found, the source machine checks its ARP cache for the default gateway IP address and corresponding MAC address-ARP resolution may be needed here for default gateway Remote ARP Resolution –Cont. • The source sends the destination IP address to the default gateway (router) in order for the router to determine which network the destination is on • The router determines whether the IP address of the destination is local or remote • If the destination is remote, the router consults its own routing table for a route to the network where the destination node is found • The router also consults its own ARP cache to determine the hardware address of the next router Remote ARP Resolution – Cont. • The source host was on a remote network, so the destination host has to consult its own routing table for a path back to the source. • It also must consult its own ARP cache as well to get the hardware address of the router that will be used to get the packet back to the source • The packet travels back through the routers to the original network where the source host is found Utilities • Arp –a • Ping • Ipconfig /all