[standards-jig] Re: My IP Address

Richard Dobson richard at dobson-i.net
Wed Sep 3 00:02:26 UTC 2003

> > Yea I agree that the IP address itself is not all that useful when
behind a
> > NAT since you will probably not be able to use it for anything, although
> > is useful if you are just trying to automatically find out if you are
> > a NAT by comparing that IP with the known IP's of your interfaces, i.e.
> > the external IP does not match one of your interface IP's then there you
> > likely behind a NAT.
> If your IP is in one of the following subnets you can expect that you
> are NATed:
>, or for RFC 1918 address space
> or for link local addresses
> If your IP is different from these networks you can expect that you are
> not NATed and that the server will respond with the same IP address. But
> that does not help you very much as you don't know if there is a
> non-NAT firewall.

Just because you are not on one of those subnets it doesnt always mean you
are not behind a NAT. You cant assume that all network admins worldwide will
have set their networks up how you think they should, NATs are not always
used simply as a method of sharing a global IP between local link addresses,
they are sometimes used as rudamentory firewalls "protecting" network
segments (of publicly routable IP addresses), now this IMO is not the right
way for people to set their networks up but I have come across some in the
past so I know from past experience that those setups exist (on some quite
large networks).

Yes it will not show the existance of a firewall in your way if it doesnt
alter your IP address (NAT/Proxy), but it is one step in knowing your
situation and what things will most likely work.

> Just from knowing if you are behind a NAT router you do not know which
> file transfers will work for you.

Of course you dont know just from that but it is a stepping stone in knowing
which things you need or do not need to try.


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