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When a network administrator uses the
process of subnetting to divide a major IP address into multiple
subnets, a new subnet mask is created. This new subnet mask is crucial
in identifying network, subnet, and host bits in an IP address. If a
host does not know the subnet mask, it may send an address mask
request to the local router. If the address of the router is known,
this request may be sent directly to the router. Otherwise, the
request will be broadcast. When the router receives the request, it
will respond with an address mask reply. This address mask reply will
identify the correct subnet mask. For example, assume that a host is
located within a Class B network and has an IP address of 172.16.5.2.
This host does not know the subnet mask, thus it broadcasts an address
mask request:
| Source address: |
172.16.5.2 |
| Destination address: |
255.255.255.255 |
| Protocol: |
ICMP = 1 |
| Type: |
Address Mask Request =
AM1 |
| Code: |
0 |
| Mask: |
255.255.255.0 |
This broadcast is received by
172.16.5.1, the local router. The router responds with the address
mask reply:
| Source address: |
172.16.5.1 |
| Destination address: |
172.16.5.2 |
| Protocol: |
ICMP = 1 |
| Type: |
Address Mask Reply
= AM2 |
| Code: |
0 |
| Mask: |
255.255.255.0 |
The frame format for the address mask
request and reply is shown in Figure
 .
Figure
shows the descriptions for each field in the address mask request
message. Note that
the same frame format is used for both the address mask request and
the reply. However, an ICMP type number of 17 is assigned to the
request and 18 is assigned to the reply.
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