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!

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 Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)
                9  Internet Protocol Version 4 (IP, IPv4)
                     9  IP Addressing
                          9  IP "Classful" (Conventional) Addressing

Previous Topic/Section
IP Reserved, Loopback and Private Addresses
Previous Page
Pages in Current Topic/Section
Next Page
Problems With "Classful" IP Addressing
Next Topic/Section

IP Multicast Addressing
(Page 1 of 2)

The vast majority of traffic on IP internetworks is of the unicast variety: one source device sending to one destination device. IP also supports multicasting, where a source device can send to a group of devices. Multicasting is not used a great deal on the Internet as a whole at the present time, mainly due to lack of widespread hardware support, so most of our focus in looking at IP is on unicast. Multicast is useful in certain circumstances, however, especially as a more efficient alternative to broadcasting. I include one summary topic on multicasting for your perusal, and also want to briefly discuss here IP addressing issues related to multicasting.

The “classful” IP addressing scheme sets aside a full one-sixteenth of the address space for multicast addresses: Class D. Multicast addresses are identified by the pattern “1110” in the first four bits, which corresponds to a first octet of 224 to 239. So, the full range of multicast addresses is from to Since multicast addresses represent a group of IP devices (sometimes called a host group) they can only be used as the destination of a datagram; never the source.

Multicast Address Types and Ranges

The 28 bits after the leading “1110” in the IP address define the multicast group address. The size of the Class D multicast address space is therefore 228 or 268,435,456 multicast groups. There is no substructure that defines the use of these 28 bits; there is no specific concept of a network ID and host ID as in classes A, B and C. However, certain portions of the address space are set aside for specific uses. Table 48 and Figure 63 show the general allocation of the Class D address space.

Table 48: IP Multicast Address Ranges and Uses

Range Start Address

Range End Address


Reserved for special “well-known” multicast addresses.

Globally-scoped (Internet-wide) multicast addresses.

Administratively-scoped (local) multicast addresses.

Note: As with the other IP address classes, the entire 32 bits of the address is always used; we are just only interested in the least-significant 28 bits because the upper four bits never change.

The bulk of the address space is in the middle multicast range, which are “normal” multicast addresses. They are analogous to the Class A, B and C unicast addresses and can be assigned to various groups.

The last address range is for administratively-scoped multicast groups. This is a fancy term for multicast groups used within a private organization; this block, representing 1/16th of the total multicast address space, is comparable to the private addresses we saw in the preceding topic. This block is also subdivided further into site-local multicast addresses, organization-local addresses and so forth.

Figure 63: IP Multicast Address Ranges and Uses

All multicast addresses begin with “1110” as shown. The “well-known” group has zeroes for the first 20 bits of the multicast group address, with 8 bits available to define 255 special multicast addresses. Multicast addresses starting with “1110 1111” are locally-scoped; all other addresses are globally-scoped (this includes addresses starting with “1110 0000” other than the 255 “well-known” addresses.)


Related Information: The concept of multicast address scope was more completely defined in IPv6, and I discuss it in more detail in the in the discussion of IPv6 multicast addresses.

Previous Topic/Section
IP Reserved, Loopback and Private Addresses
Previous Page
Pages in Current Topic/Section
Next Page
Problems With "Classful" IP Addressing
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.