blank.gif (43 bytes)

Church Of The
Swimming Elephant

An Example Connected: An Internet Encyclopedia
An Example

Up: Connected: An Internet Encyclopedia
Up: Requests For Comments
Up: RFC 826
Prev: Network monitoring and debugging
Next: Related issue

An Example

An Example 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 48.bit 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.

Next: Related issue

Connected: An Internet Encyclopedia
An Example


Protect yourself from cyberstalkers, identity thieves, and those who would snoop on you.
Stop spam from invading your inbox without losing the mail you want. We give you more control over your e-mail than any other service.
Block popups, ads, and malicious scripts while you surf the net through our anonymous proxies.
Participate in Usenet, host your web files, easily send anonymous messages, and more, much more.
All private, all encrypted, all secure, all in an easy to use service, and all for only $5.95 a month!

Service Details

Have you gone to church today?
All pages ©1999, 2000, 2001, 2002, 2003 Church of the Swimming Elephant unless otherwise stated
Church of the Swimming Elephant©1999, 2000, 2001, 2002, 2003 is a wholly owned subsidiary of Packetderm, LLC.

Packetderm, LLC
210 Park Ave #308
Worcester, MA 01609