Name

Address Mask Request and Address Mask Reply

Synopsis

The ICMP Address Mask Request and Address Mask Reply query messages provide a host with the ability to determine the subnet mask in use on the local network. This is achieved by sending an Address Mask Request query message to the local broadcast address of 255.255.255.255, which is then responded to with an Address Mask Reply message.

ICMP Message Type

The Message Type for Address Mask Request is 17. The Message Type for Address Mask Reply is 18.

ICMP Message Codes

Neither the Address Mask Request nor Address Mask Reply query messages use any Codes, and this field must be zero.

Defined In

RFC 950.

Additional Fields

Both the Address Mask Request and Address Mask Reply query messages use the same message structure. The additional fields used by these messages are shown in Table 5.14.

Table 5.14. Additional Fields in Address Mask Request and Address Mask Reply Query Messages

Field

Bytes

Description

Identifier

2

Provides a session identification number for the request and reply messages

Sequence Number

2

Provides a counter for requests and replies, allowing multiple messages to be used with a single Identifier

Subnet Mask

4

The subnet mask in use on the local network

These fields are explained below:

  • The Identifier field provides a kind of session identification number for the Address Mask Request and Address Mask Reply query messages being passed between various hosts. For example, if Ferret issues multiple Address Mask Request queries as part of ...

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