Please Whitelist This Site?

I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :)

If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads.

If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK.

Thanks for your understanding!

Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide


NOTE: Using software to mass-download the site degrades the server and is prohibited.
If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The Book is Here... and Now On Sale!

Read offline with no ads or diagram watermarks!
The TCP/IP Guide

Custom Search







Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Network Interface / Internet "Layer Connection" Protocols
           9  Address Resolution and the TCP/IP Address Resolution Protocol (ARP)
                9  TCP/IP Address Resolution Protocol (ARP)

Previous Topic/Section
ARP Caching
Previous Page
Pages in Current Topic/Section
1
2
Next Page
TCP/IP Address Resolution For IP Multicast Addresses
Next Topic/Section

Proxy ARP
(Page 1 of 2)

ARP was designed to be used by devices that are directly connected on a local network. Each device on the network should be capable of sending both unicast and broadcast transmissions directly to each other one. Normally, if device A and device B are separated by a router, they would not be considered local to each other. Device A would not send directly to B or vice-versa; they would send to the router instead at layer two, and would be considered “two hops apart” at layer three.

Why Proxy ARP Is Needed

In contrast to the normal situation, in some networks there might be two physical network segments connected by a router that are in the same IP network or subnetwork. In other words, device A and device B might be on different networks at the data link layer level, but on the same IP network or subnet. When this happens, A and B will each think the other is on the local network when they look to send IP datagrams.

In this situation, suppose that A wants to send a datagram to B. It doesn't have B's hardware address in the cache, so it begins an address resolution. When it broadcasts the ARP Request message to get B's hardware address, however, it will quickly run into a problem: B is in fact not on A's local network. The router between them will not pass A's broadcast onto B's part of the network, because routers don't pass hardware-layer broadcasts. B will never get the request and thus A will not get a reply containing B’s hardware address.


Previous Topic/Section
ARP Caching
Previous Page
Pages in Current Topic/Section
1
2
Next Page
TCP/IP Address Resolution For IP Multicast Addresses
Next Topic/Section

If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!
Donate $2
Donate $5
Donate $10
Donate $20
Donate $30
Donate: $



Home - Table Of Contents - Contact Us

The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005

Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.