Birgit Bonham: Prospect High School ARP….or What’s your MAC address?

Birgit Bonham: Prospect High School Packet Transmission NICs transmit packets from Source Destination

Birgit Bonham: Prospect High School Each host on the network checks packet to see if – the MAC address matches its own – it’s a broadcast to all hosts

Birgit Bonham: Prospect High School Where does MAC address checking occur ? TCP/IP - network interface layer OSI - Data Link Layer MAC address uniquely identifies a specific device on the network. IP address identifies a specific host on a specific network Both MAC and IP addresses are required

Birgit Bonham: Prospect High School Routed Protocols On TCP/IP networks Packets of routed protocols Contain Internetwork layer addressing info –Allows user traffic to be directed from one network to another Define structure and use of fields within packet Types of routed protocols: –ARP –RARP

Birgit Bonham: Prospect High School ARP Address Resolution Protocol –Maps IP addresses to MAC addresses –ARP tables are maintained in networking devices (I.e. routers) Also called ARP cache Maintained in RAM May be modified by network administrators – but not typical – usually maintained by device.

Birgit Bonham: Prospect High School ARP request If computer cannot locate IP to MAC address mapping in its ARP table –Must obtain correct mapping –Initiates an ARP request –Source computer broadcasts ARP request to all hosts on local segment –Each device checks if packet is destined for its own IP address – if not – ignores If yes - responds

Birgit Bonham: Prospect High School ARP frame Logical configuration Frame header MAC header Destination Ff-ff-ff-ff-ff-ff Source BC IP Header Destination Source Frame Header ARP Request Message What is your MAC address?

Birgit Bonham: Prospect High School Real packets: 28 octets –Hardware types –Protocol types –Message lengths

Birgit Bonham: Prospect High School Source host does not know the MAC address of the destination. ARP request frame is a broadcast to all MAC addresses MAC header Destination Ff-ff-ff-ff-ff-ff Source BC

Birgit Bonham: Prospect High School All devices on the local segment will investigate the frame to check if their own IP address matches the destination IP address. IP Header Destination Source ME?

Birgit Bonham: Prospect High School If no – discard the packet If yes: –Reads rest of packet –Returns ARP reply ARP reply –Supplies MAC address of the host. Source: Arp Request Hey –my IP matches My MAC address no

Birgit Bonham: Prospect High School ARP Cache Life Source checks its local ARP cache before sending request  Must contain current information – else data packets could be routed to the wrong host. ARP table directs data packets to a specific network card based on mapping  Mapping must be correct

Birgit Bonham: Prospect High School –Host addresses can change … so old ARP table mappings can cause misrouted packets. To avoid problems –Networking devices place a TTL (time-to-live) on ARP entries. Ex. Microsoft operating systems use a two-to ten-minute TTL. Using TTLs the operating system ensures that ARP cache is never outdated for more than 10 min Process of removing ARP entries from an ARP table is called aging out. In addition, devices on network replace ARP entries whenever they receive new information. –Ex. If computer receives packet from host with new info, old info is replaced.

Birgit Bonham: Prospect High School RARP Reverse Address Resolution Protocol Similar to ARP (used to bind MAC addresses to IP addresses) Used primarily by diskless workstations. –NIC with MAC address –No IP addresses

Birgit Bonham: Prospect High School Diskless Workstations –Don’t have hard drives that hold IP configurations –Must obtain IP address each time they boot up. –Client’s IP config must be stored on a RARP server. –RARP server maintains table of IP address to MAC address mappings for RARP clients. –During boot-up RARP clients call RARP server to obtain their IP config information.

Birgit Bonham: Prospect High School RARP Frame vs. ARP Frame Frame Header MAC header Destination Ff-ff-ff-ff-ff-ff Source C IP Header Destination Source ARP Request Message What is your MAC address? Frame Header MAC header Destination Ff-ff-ff-ff-ff-ff Source C IP Header Destination Source ARP Request Message What is my IP address?

Birgit Bonham: Prospect High School More info…. Note: IP header does not have specific destination or source IP –Source does not know which device is RARP server must broadcast to all devices on the segment

Birgit Bonham: Prospect High School RARP Client RARP client receives RARP reply: –Configures its IP networking –Copies its IP address config info into local RAM –After settings have been received, client can use MAC and IP address to send packets on the network

Birgit Bonham: Prospect High School When diskless workstation reboots, or is shut down –IP config is lost –Each time workstation reboots it must obtain IP config from RARP server.

Birgit Bonham: Prospect High School Compare ARP & RARP Similarities Both are concerned with mapping IP to MAC addresses Both use same packet format Both use broadcast addresses for their task.

Birgit Bonham: Prospect High School Differences ARP - obtains MAC address of other clients by using an IP address RARP – obtains its own (local host) IP address by using its own (local host’s) MAC address ARP packets are broadcast on local network by using broadcast MAC address (ff-ff-ff-ff-ff-ff) RARP uses broadcast IP address ( ) as well as broadcast MAC address (ff-ff-ff-ff-ff-ff)

Birgit Bonham: Prospect High School ARP table maintained by local host RARP table maintained by RARP server ARP reply is used by local host to update its ARP table and to send packets to destination. RARP reply is used to configure the IP protocol of a local host.