Question about creating iRules

Well, the first thing I'd not is that you're missing an ending bracket on both of your matchclass lines.

Assuming your classes are set up properly to match the IP info, the rule would read:

when CLIENT_ACCEPTED {
  set ckname NETWORK_ALIAS
  if { [matchclass $::Alias_Subnet contains [IP::client_addr] ] } {
    set ckvalue ATLAS
  } elseif { [matchclass $::Test_Subnet contains [IP::client_addr] ] } {
    set ckvalue TEST
  } else {
    set ckvalue UNKNOWN
  }
}
when HTTP_REQUEST {
  HTTP::header insert $ckname $ckvalue
}

There's a great post on matching network masks here, as well: Click here

Happy Coding,

Colin