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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 Mobility Support (Mobile IP)

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Mobile IP Addressing: Home and "Care-Of" Addresses
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Mobile IP Home Agent Registration and Registration Messages
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Mobile IP Agent Discovery, and Agent Advertisement and Solicitation Messages
(Page 5 of 5)

Prefix-Lengths Extension

This is an optional extension that tells a mobile node the prefix length(s) of the router address(es) contained in the regular portion of the Agent Advertisement message; that is, the Router Address field in the regular Router Advertisement part of the message. The prefix length is another term for the number of bits in an IP address is the network ID, so this tells the node the network ID in each of the router's addresses

The format of the Prefix-Lengths Extension is shown in Table 82 and also in Figure 131.

Table 82: Mobile IP Prefix-Lengths Extension Format

Field Name

Size (bytes)




Extension Type: Identifies the Agent Advertisement extension type. For the Prefix-Lengths Extension, it is set to 19.



Length: Length of the extension in bytes, excluding the Type and Length fields. Thus, it is equal to the number of prefix lengths (since each takes 1 byte).

Prefix Lengths

Variable (1 per length)

Prefix Lengths: One prefix length number for each router address in the regular, Router Advertisement portion of the Agent Advertisement.

Figure 131: Mobile IP Prefix-Lengths Extension Format

This extension appears after the normal fields of a Router Advertisement message, as shown in Figure 148.


One-Byte Padding Extension

Some implementations require ICMP messages to be an even number of bytes, so a byte of padding is needed. (The One-Byte Padding Extension is just a single byte of all zeroes. No tables or figures needed for that!)

Key Concept: Mobile IP agent discovery is the process by which a mobile node determines where it is located and establishes contact with a home or foreign agent. Routers that can function as agents regularly send Agent Advertisement messages to indicate their capabilities, which are modified versions of regular Router Advertisements. A mobile node can also send an Agent Solicitation to request the sending of an Advertisement, which is the same as regular Router Solicitation.

Note: Mobile IP does not include any provisions for authentication of Agent Advertisement and Agent Solicitation messages. They may be authenticated using IPSec, of course, if that has been implemented.

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Mobile IP Home Agent Registration and Registration Messages
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