Two IP address for WAN

My Internet ISP is my cable company.  So, i connect through a cable 
modem.  I setup a m0n0wall firewall behind the cable modem.  I gets it's 
IP address via DHCP.  When I go to the webGUI Configuration page under 
Status|Interfaces it lists the WAN interface as indicated below:

Status                up
MAC address   00:xx:xx:xx:xx:xx
IP address         XX.YY.ZZZ.153
Subnet mask      255.255.248.0
Gateway            XX.YY.ZZZ.1

Why the two addresses?  Gateway and IP.

I can ping the gateway from outside the LAN but can't ping the IP 
address.  What gives?

Any help is appreciated.

Later,
-pvaughan