Nmap Development: help for port scanning on firewalls and routers

[Joseph Taylor <joseph_taylor_mail () yahoo com>]

Hello,

I am planning to perform port scanning for routers and
firewalls security on my network.
My questions are about what benefits I will obtain by
running a port scanning tool (NMAP, ISS, etc..). 

Will there be any difference in the results whether I
use this tool inside my network (from an internal IP) 
or outside my network.  

What's the aim of running this tool from internal
network?

Do I need to make port scanning over devices which are
not accessible outside my network from internet and
are only accessible from internal network? (such as
WAN routers and LAN&WAN firewalls.)

I was planning to run "NMAP", do you think it
satisfies my aim?

Can anyone give me a brief explanation?

Thanks, 


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