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 Internet Layer (OSI Network Layer) Protocols
           9  Internet Control Message Protocol (ICMP/ICMPv4 and ICMPv6)
                9  ICMP Message Types and Formats
                     9  ICMP Version 4 (ICMPv4) Informational Message Types and Formats

Previous Topic/Section
ICMPv4 Router Advertisement and Router Solicitation Messages
Previous Page
Pages in Current Topic/Section
1
2
Next Page
ICMPv4 Traceroute Messages
Next Topic/Section

ICMPv4 Address Mask Request and Reply Messages
(Page 1 of 2)

When the Internet Protocol was first developed, IP addresses were based on a simple two-level structure, with a network identifier and host identifier. To provide more flexibility, a technique called subnetting was soon developed that expands the addressing scheme into a three-level structure, with each address containing a network identifier, subnet identifier and host identifier. The subnet mask is a 32-bit number that tells devices (and users) which bits are part of the subnet identifier, as opposed to the host identifier. All of this is described in considerable detail in the section on IP addressing.

To function properly in a subnetting environment, each host must know the subnet mask that corresponds to each address it is assigned—without the mask it cannot properly interpret IP addresses. Just like determining the identity of a local router, a host can be informed of the local network's subnet mask either manually or automatically. The manual method is to simply to have the subnet mask manually assigned to each host. The automatic method makes use of a pair of ICMP messages for subnet mask determination, which were defined in RFC 950, the same standard that defined subnetting itself.

To use this method, a host sends an Address Mask Request message on the local network, usually to get a response from a router. If it knows the address of a local router it may send the request directly (unicast), but otherwise will broadcast it to any listening router. A local router (or other device) will hopefully receive this message and respond back with an Address Mask Reply containing the subnet mask for the local network. This process is somewhat similar to the mechanism used by a host to solicit a router to respond with a Router Advertisement, except that routers do not routinely send subnet mask information; it must be requested.


Previous Topic/Section
ICMPv4 Router Advertisement and Router Solicitation Messages
Previous Page
Pages in Current Topic/Section
1
2
Next Page
ICMPv4 Traceroute Messages
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.