ARP ( Address Resolution Protocol )

• ARP request


• ARP reply


• RARP request


• RARP reply

• Hardware type

16 bits.


Value	Description
1	Ethernet.
2	Experimental Ethernet.
3	Amateur Radio AX.25.
4	Proteon ProNET Token Ring.
5	Chaos.
6	IEEE 802.
7	ARCNET.
8	Hyperchannel.
9	Lanstar.
10	Autonet Short Address.
11	LocalTalk.
12	LocalNet (IBM PCNet or SYTEK LocalNET).
13	Ultra link.
14	SMDS.
15	Frame Relay.
16	ATM, Asynchronous Transmission Mode.
17	HDLC.
18	Fibre Channel.
19	ATM, Asynchronous Transmission Mode.
20	Serial Line.
21	ATM, Asynchronous Transmission Mode.
22	MIL-STD-188-220.
23	Metricom.
24	IEEE 1394.1995.
25	MAPOS.
26	Twinaxial.
27	EUI-64.
28	HIPARP.
29	IP and ARP over ISO 7816-3.
30	ARPSec.
31	IPsec tunnel.
32	Infiniband.
33	CAI, TIA-102 Project 25 Common Air Interface.



• Protocol type

16 bits.


Value	Description
0x800	IP.



• Hardware address length

8 bits. Length of the hardware address in bytes.


• Protocol address length

8 bits. Length of the protocol address in bytes.


• Opcode

16 bits.


Value	Description	References
1	Request.
2	Reply.
3	Request Reverse.	In the case where the sender wishes to determine his own protocol address will be the hardware address of the sender.
4	Reply Reverse.	If a system were to use both ARP and RARP, then the inclusion of the valid protocol-hardware address pair may eliminate the need for a subsequent ARP request.
5	DRARP Request.	DRARP Request packets have the same format as REVARP Request packets, except for the operation code.
6	DRARP Reply.	DRARP Reply packets have the same format, opcode excepted, as REVARP Reply packets.
7	DRARP Error.	DRARP Error packets may also be sent in response to DRARP Request.
8	InARP Request.	A station may put the requester's protocol address/hardware address mapping into its ARP cache as it would any ARP request.
9	InARP Reply.	When the requesting station receives the InARP reply, it may complete the ARP table entry and use the provided address information.
10	ARP NAK.	The ARP NAK response is an extension to the ARMARP protocol and is used to improve the robustness of the ATMARP server mechanism.
11	MARS Request.
12	MARS Multi.
13	MARS MServ.
14	MARS Join.
15	MARS Leave.
16	MARS NAK.
17	MARS Unserv.
18	MARS SJoin.
19	MARS SLeave.
20	MARS Grouplist Request.
21	MARS Grouplist Reply.
22	MARS Redirect Map.
23	MAPOS UNARP.



• Source hardware address

Variable length.


• Source protocol address

Variable length.


• Destination hardware address

Variable length.


• Destination protocol address

Variable length

Let there exist machines X and Y that are on the same 10Mbit Ethernet cable. They have 

Ethernet address EA(X) and EA(Y) and DOD Internet addresses IPA(X) and IPA(Y).  Let 

the Ethernet type of Internet be ET(IP). Machine X has just been started, and sooner or 

later wants to send an Internet packet to machine Y on the same cable. X knows that it 

wants to send to IPA(Y) and tells the hardware driver (here an Ethernet driver) IPA(Y). 

The driver consults the Address Resolution module to convert  into a 48bit Ethernet 

address, but because X was just started, it does not have this information. It throws 

the Internet packet away and instead creates an ADDRESS RESOLUTION packet with



          (ar$hrd) = ares_hrd$Ethernet

          (ar$pro) = ET(IP)

          (ar$hln) = length(EA(X))

          (ar$pln) = length(IPA(X))

          (ar$op)  = ares_op$REQUEST

          (ar$sha) = EA(X)

          (ar$spa) = IPA(X)

          (ar$tha) = don't care

          (ar$tpa) = IPA(Y)



and broadcasts this packet to everybody on the cable.



Machine Y gets this packet, and determines that it understands the hardware type 

(Ethernet), that it speaks the indicated protocol (Internet) and that the packet is 

for it ((ar$tpa)=IPA(Y)). It enters (probably replacing any existing entry) the 

information that  maps to EA(X). It then notices that it is a request, so it 

swaps fields, putting EA(Y) in the new sender Ethernet address field (ar$sha), sets 

the opcode to reply, and sends the packet directly (not broadcast) to EA(X). At this 

point Y knows how to send to X, but X still doesn't know how to send to Y.



Machine X gets the reply packet from Y, forms the map from  to EA(Y), notices the 

packet is a reply and throws it away. The next time X's Internet module tries to send 

a packet to Y on the Ethernet, the translation will succeed, and the packet will 

(hopefully) arrive. If Y's Internet module then wants to talk to X, this will also 

succeed since Y has remembered the information from X's request for Address Resolution.

	  

■ Parent layer
■ Child layer

WAN Frame Relay Ethernet VINES

ARP

Request Response