If this is taking 1.5 hours, then whatever system you are using can't be that efficient. Assuming you're looking for a particular port to be open, you can do this quickly with NMAP. Here is an example that looks for machines with port 80 (http) open: 
% nmap X.X.X.0/24 -p 80 -T Insane -n
The 1 scanned port on  (X.X.X.137) is: closed
The 1 scanned port on  (X.X.X.146) is: closed
The 1 scanned port on  (X.X.X.147) is: closed
Interesting ports on  (X.X.X.152):
Port       State       Service
80/tcp     open        http                    
:

[download]
So you get this output quickly and easily. If you want, you can choose XML output using -oX and then use XML::Parser to get the goods.

As the root user you can engage the -PS option which uses SYN instead of ACK, the feature you were asking how to implement.

Even over 300-baud dial-up, NMAP should be able to finish in several minutes for a /28, which is all of 30 hosts. 30. That's not a lot.

In reply to Re^3: Low Level Network Connections by tadman
in thread Low Level Network Connections by Anonymous Monk

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