is there a way to bypass a firewall to detect the internal network. using tools like nmap and nessus

here is what i get when i try to scan a target that uses a firewall

Starting nmap 3.48 ( http://www.insecure.org/nmap/ ) at 2003-12-26 21:44 EST
Note: Host seems down. If it is really up, but blocking our ping probes, try -P0Nmap run completed -- 1 IP address (0 hosts up) scanned in 16.064 seconds


yet when i ping the target i get this

bash-2.05b# ping 204.118.xxx.xxx
PING 204.118.xxx.xxx (204.118.xxx.xxx) 56(84) bytes of data.
64 bytes from 204.118.xxx.xxx: icmp_seq=5760 ttl=50 time=62.3 ms
64 bytes from 204.118.xxx.xxx: icmp_seq=5761 ttl=50 time=64.3 ms
64 bytes from 204.118.xxx.xxx: icmp_seq=5762 ttl=50 time=62.3 ms
64 bytes from 204.118.xxx.xxx: icmp_seq=5763 ttl=50 time=61.3 ms