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". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||^$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!

The whole site in one document for easy reference!
The TCP/IP Guide

Custom Search

Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  TCP/IP Key Applications and Application Protocols
           9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)
                9  TCP/IP World Wide Web (WWW, "The Web") and the Hypertext Transfer Protocol (HTTP)
                     9  TCP/IP Hypertext Transfer Protocol (HTTP)
                          9  HTTP Message Headers

Previous Topic/Section
HTTP Response Headers
Previous Page
Pages in Current Topic/Section
Next Page
HTTP Entities, Transfers, Coding Methods and Content Management
Next Topic/Section

HTTP Entity Headers
(Page 3 of 3)


Specifies the media type and subtype of the entity, in a manner very similar to how this header is used in MIME. See the topic describing HTTP entities and Internet media types for a full discussion.


Specifies a date and time after which the entity in the message should be considered “stale”. This may be used to identify certain entities that should be held in HTTP caches for longer or shorter periods of time than usual.

This header is ignored if a Cache-Control header containing the “max-age” directive is present in the message.


Indicates the date and time when the server believes the entity was last changed. This header is often used to determine if a resource has been modified since it was last retrieved. For example, suppose a client machine already contains a copy of a very large file that was obtained two months ago, and its user wants to check if an update to the file is available. The client can send a HEAD request for the file, and compare the value of the returned Last-Modified header to the date of the copy of the file it already has. Then it needs only to request the entire file if it has changed.

Note the use of the word “believe” in the description above. The reason for this wording is that the server cannot always be certain of the time that a resource was modified. With files this is fairly simple—it is usually the last-modified time stored for the file by the operating system. For other more complex resources such as database records or virtual objects, however, it may be more difficult to ascertain when the last change occurred to a particular piece of information. In the case of dynamically-generated content, the Last-Modified date/time may be the same as that of the message as a whole, as specified in the Date field.

Key Concept: HTTP entity headers appear in either request or response messages that carry an entity in the message body. They describe the nature of the entity, including its type, language and encoding, to facilitate the proper processing and presentation of the entity by the device receiving it.


Previous Topic/Section
HTTP Response Headers
Previous Page
Pages in Current Topic/Section
Next Page
HTTP Entities, Transfers, Coding Methods and Content Management
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 (
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.