iRule to manipulate designation content based off ip/subnet

Have you tried logging any data to see what's wrong with your logic? That should always be the first step in diagnosing a problem. One thing I noticed is that you'll want to use the HTTP::redirect command instead of "redirect to".

when CLIENT_ACCEPTED {
  log local0. "client address: [IP::client_addr]"
  if { [IP::addr [IP::client_addr] equals 156.74.138.63] } {
    log local0. "matched address 156.74.138.63"
    pool GWWEB-HTTP
  } elseif { [IP::addr [IP::client_addr] equals 156.74.12.223] } {
    log local0. "matched address 156.74.12.223"
    pool GWWEB-HTTP
  } elseif { [IP::addr [IP::client_addr] equals 156.74.12.222] } {
    log local0. "matched address 156.74.12.222"
    pool GWWEB-HTTP
  } elseif {not[IP::addr [IP::client_addr] equals 156.74.16.0/24] } {
    log local0. "matched subnet 156.74.16.0/24"
    set issue_redirect 1
  } else { 
    log local0. "didn't match any addresses"
    pool GWWEB-HTTP
  }
}
when HTTP_REQUEST {
  log local0. "issue_redirect: $issue_redirect"
  if { $issue_redirect == 1 } {
    log local0. "issuing redirect..."
    HTTP::redirect "http://inweb.ci.seattle.wa.us/ccss/noaccess.asp"
  }
}

Run your traffic through this and then look at the /var/log/ltm file. It should point you right to your problem. Make sure you comment out or remove the log statements when you move this into production.

-Joe