At a loss figuring out if an IP is on LAN or INET

[Terry Olsen]

I'm writing an app that communicates with computers both inside and outside
my router. So I need to determine by the remote host's IP address if I need
to send them my LAN IP or my Internet IP. Someone suggested AND'ing the IP
and my Subnet Mask but I come up with this:

My IP: 192.168.0.14
Mask: 255.255.255.0

Result: 192.168.0.0

Should I just compare the first two octets? Or are there situations where
this would provide incorrect results?

[Cor Ligthert [MVP]]

Terry,

AFAIK are there three ranges reserved for internal use

10.x.x.x
150.x.x.x
192.168.x.x

If I am not completely wrong than the first two should therefore be enough.

Cor

"Terry Olsen" <to******@hotma il.com> schreef in bericht
news:ug******** ******@TK2MSFTN GP05.phx.gbl...

I'm writing an app that communicates with computers both inside and
outside my router. So I need to determine by the remote host's IP address
if I need to send them my LAN IP or my Internet IP. Someone suggested
AND'ing the IP and my Subnet Mask but I come up with this:

My IP: 192.168.0.14
Mask: 255.255.255.0

Result: 192.168.0.0

Should I just compare the first two octets? Or are there situations where
this would provide incorrect results?

[Cor Ligthert [MVP]]

Terry,

Sorry when I had sent it I realized me that it was not the 150 so I searched
for it in Google.

10.x.x.x
172.16.x.x - 172.31.x.x
192.168.x.x

Cor

[Grumpy Aero Guy]

corrrect you are.

Testing against the forst two octets should be sufficient to determine
routable vs. non routable.... aka internal/external.

NOW, if an internal network is configured to have an IP scheme with ROUTABLE
IPs ( which CAN be done --- WHY? I wouldn't know)... that would make it a
bit touchy.

--
Grumpy Aero Guy

"Cor Ligthert [MVP]" <no************ @planet.nl> wrote in message
news:%2******** ********@TK2MSF TNGP03.phx.gbl. ..

Terry,

AFAIK are there three ranges reserved for internal use

10.x.x.x
150.x.x.x
192.168.x.x

If I am not completely wrong than the first two should therefore be
enough.

Cor

"Terry Olsen" <to******@hotma il.com> schreef in bericht
news:ug******** ******@TK2MSFTN GP05.phx.gbl...

I'm writing an app that communicates with computers both inside and
outside my router. So I need to determine by the remote host's IP address
if I need to send them my LAN IP or my Internet IP. Someone suggested
AND'ing the IP and my Subnet Mask but I come up with this:

My IP: 192.168.0.14
Mask: 255.255.255.0

Result: 192.168.0.0

Should I just compare the first two octets? Or are there situations where
this would provide incorrect results?

[Simon Verona]

The correct way is to AND both the PC's own IP address and the destination
IP address each with the subnet mask... If the result is the same, then
they are on the same subnet of the network.

Out of interest the result of AND'ing the ip address against the subnet mask
produces what is called the "Network Address". For class B addresses (such
as your example) this is simply the same as the first two octects followed
by 2 zeros, but this may not always be the case...

HTH
Simon

--
=============== =============== ==
Simon Verona
Dealer Management Service Ltd
Stewart House
Centurion Business Park
Julian Way
Sheffield
S9 1GD

Tel: [phone]
Fax: [phone]

"Grumpy Aero Guy" <[e-mail] g> wrote in message
news:UZ******** *********@torna do.ohiordc.rr.c om...

corrrect you are.

Testing against the forst two octets should be sufficient to determine
routable vs. non routable.... aka internal/external.

NOW, if an internal network is configured to have an IP scheme with
ROUTABLE IPs ( which CAN be done --- WHY? I wouldn't know)... that would
make it a bit touchy.

--
Grumpy Aero Guy

"Cor Ligthert [MVP]" <no************ @planet.nl> wrote in message
news:%2******** ********@TK2MSF TNGP03.phx.gbl. ..

Terry,

AFAIK are there three ranges reserved for internal use

10.x.x.x
150.x.x.x
192.168.x.x

If I am not completely wrong than the first two should therefore be
enough.

Cor

"Terry Olsen" <to******@hotma il.com> schreef in bericht
news:ug******** ******@TK2MSFTN GP05.phx.gbl...

I'm writing an app that communicates with computers both inside and
outside my router. So I need to determine by the remote host's IP
address if I need to send them my LAN IP or my Internet IP. Someone
suggested AND'ing the IP and my Subnet Mask but I come up with this:

My IP: 192.168.0.14
Mask: 255.255.255.0

Result: 192.168.0.0

Should I just compare the first two octets? Or are there situations
where this would provide incorrect results?

[aaron.kempf]

no please-- give more detail

[david]

On 2006-04-28, Terry Olsen <to******@hotma il.com> wrote:

I'm writing an app that communicates with computers both inside and outside
my router. So I need to determine by the remote host's IP address if I need
to send them my LAN IP or my Internet IP. Someone suggested AND'ing the IP
and my Subnet Mask but I come up with this:

My IP: 192.168.0.14
Mask: 255.255.255.0

Result: 192.168.0.0

Should I just compare the first two octets? Or are there situations where
this would provide incorrect results?

I'm curious, why do you need to do this? The remote host should already
know how to respond to you. If you're sending an IP to a third host
that distributes the IP (like many P2P networks), then using the IP
address to determine local vs. remote isn't going to help you, because
you'll have conflicts with identical addresses.

[Terry Olsen]

> I'm curious, why do you need to do this? The remote host should already

know how to respond to you. If you're sending an IP to a third host
that distributes the IP (like many P2P networks), then using the IP
address to determine local vs. remote isn't going to help you, because
you'll have conflicts with identical addresses.

There should not be any identical addresses. I am writing a P2P application
that communicates via UDP (as an experiment, I know UDP isn't reliable for a
chat application). I want to be able to communicate with other peers both
inside and outside my router. Since there is no TCP connection from which to
read the remote IP address, when one peer sends a message to another, it
also sends the return IP address. So I need to be able to determine if the
peer is inside or outside my router so I know which IP address to send as
the return address. I won't be sending my "192.168.x. x" address to anyone
outside my router, and I should not received a private IP from anyone
outside my router.

I believe I understand now how to get the "network address" by AND'ing the
Subnet Mask with with each IP and comparing the results. I'm going to try
that.

[Chris Chilvers]

On Fri, 28 Apr 2006 11:26:44 GMT, "Grumpy Aero Guy" <[e-mail] g> wrote:

corrrect you are.

Testing against the forst two octets should be sufficient to determine
routable vs. non routable.... aka internal/external.

NOW, if an internal network is configured to have an IP scheme with ROUTABLE
IPs ( which CAN be done --- WHY? I wouldn't know)... that would make it a
bit touchy.

The computers could have external static IPs that all lie on the same subnet. This most likely a few servers beloning to
one company that have brought a block of ips, etc. They now can have both their 'internal' network between each other
(internal being that they all lie on the same side of a gateway) but at the same time they are external (probaly have a
firewall though, at least I'd hope they do).

[david]

On 2006-04-29, Terry Olsen <to******@hotma il.com> wrote:

I'm curious, why do you need to do this? The remote host should already
know how to respond to you. If you're sending an IP to a third host
that distributes the IP (like many P2P networks), then using the IP
address to determine local vs. remote isn't going to help you, because
you'll have conflicts with identical addresses.
There should not be any identical addresses. I am writing a P2P application
that communicates via UDP (as an experiment, I know UDP isn't reliable for a
chat application). I want to be able to communicate with other peers both
inside and outside my router. Since there is no TCP connection from which to
read the remote IP address, when one peer sends a message to another, it
also sends the return IP address.

I don't see where the TCP vs. UDP issue comes in. Both have the source
address in the packet.
So I need to be able to determine if the
peer is inside or outside my router so I know which IP address to send as
the return address. I won't be sending my "192.168.x. x" address to anyone
outside my router, and I should not received a private IP from anyone
outside my router.
Maybe I just don't understand the problem. I can sorta see issue in
p2p swarming apps like BitTorrent or Kazaa where a third server needs
to keep lists of distributable IP addresses and the sender address might
not match the distributable address.

In this case though, it doesn't look like that's happening. So why
can't a node receiving a packet simply reply to the sender, without the
issue of embedding a user-defined IP address in the data.
I believe I understand now how to get the "network address" by AND'ing the
Subnet Mask with with each IP and comparing the results. I'm going to try
that.

That's going to break on any internal network with routing. In other
words, on anything larger than a very trivial home network.

This is kinda what I'm getting at. Checking the subnet mask will
answer the question you asked, i.e., whether a transmission went through
a router. But the question you want seems to be whether a tranmission
went through a NAT router, which is a completely different question.

Of course, I'm not sure why you need the second question either...