So, a device uses its routing table to determine the destination for a frame. The "destination" from the routing table is either an IP address or the awareness that the frame should be delivered directly. Furthermore, the destination may be perceived to be on the same subnet to begin with and the routing table isn't even consulted. In every case, when a station (be it an end-node or a router) wants to send a frame to some particular IP destination it must first figure out what data link destination address will be used in the frame. This is referred to as the process of Address Resolution.
There are two methods used for address resolution in IP networks. The first method, used in IP Version 4, uses ARP (Address Resolution Protocol); this method has been used since the 1970's and is considered the standard. The new method, in IP Version 6 (IPng, "IP Next Generation"; the final drafts of which came out in January, 1995) which uses ICMP (Internet Control Message Protocol) for address resolution.
ICMP has, as a protocol, been in use since the 1970"s but its use in address resolution is brand new with IP Version 6. In 1996, the deployment of IPng (Version 6) is limited and it will surely be a number of months before the implications of ICMP resolution become significant to most networks. Consequently, in this document only the ARP method of address resolution will be examined in detail.
An ARP frame has four significant fields relative to our discussion. These are:
Some slight reflection will lead you to correctly conclude that all four of these fields are not filled in by the sender of an ARP frame. If I want to know the data link address being used by 220.127.116.11 then I will put 18.104.22.168 in the Target Protocol Address field and, typically, I will set the Target Hardware Address field to zero's; I don't know the hardware address of 22.214.171.124 - That's what I'm looking for.
I send this ARP Command frame to the Ethernet or Token-Ring broadcast destination - everyone on the cable hears the frame. If 126.96.36.199 hears the frame then it responds with an ARP Reply. In the ARP Reply, 188.8.131.52 supplies its data link (hardware) address. In this way I know the association between the IP address and the data link address; I have resolved the IP address.
The association between IP address and data link address is recorded in a temporary memory table referred to as the ARP Cache. The ARP Cache tells a station what data link destination address to use for every IP destination address.
When a station first boots up it must ARP for the IP address of the Default Gateway. Additionally, prior to sending to any destination IP address, there must be an entry in the ARP Cache associated with that destination IP address. The ARP Cache is dynamic. Entries age out after just a few seconds if they are unused. Consequently, during a typical conversation a station ARPUs for the destination once at the beginning and then the entry remains in the ARP Cache for the life of the connection (assuming the connection is active).
The two tables (the Routing Table and the ARP Cache) form the basis for frame transmission and forwarding. The address mask serves as the guide for interpreting the IP address. The Default Gateway is the destination specified in the routing table to use when no other destination can be ascertained. The idea of a default destination has no corresponding behavior in the ARP Cache.
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