Group masks are used to aggregate addresses based on subnet. A group mask has the following form:

<Address>/<Scope>/<Mask>

Scope and Mask are two subnet masks that are used in conjunction with Address to specify an grouping policy. If an address falls within the subnet specified by Address/Scope, then Mask is applied to the address to extract a subnet.

If more than one group mask is given in a list, then the mask with the narrowest scope is applied. If no masks match, then the address will pass through unchanged.

The following examples illustrate the use of group masks.

groupMask=10.0.0.0/255.0.0.0/255.255.255.0 (or more compactly groupMask=10.0.0.0/8/24) applies the subnet mask 255.255.255.0 to all addresses in the 10.* range.

groupMask=10.23.0.0/16/25,10.0.0.0/8/24 applies a 25 bit subnet mask to addresses in the 10.23.* subnet while applying a 24 bit mask to all other addresses in the 10.* range.

A variant of the group mask allows address ranges to be associated with names:

groupMask=10.0.0.0/8/internal,0.0.0.0/0/external would substitute the string "internal" for any address in the 10.* range and the string "external" would be substituted for all other addresses.

Finally, group masks can be automatically constructed using the site:zone:subnet information in the Traffic Server configuration file. The following automatic groupMask options are available: