Is is possible for someone to be able to tell if one IP is in contact w/ another IP?

Answer 1

If you want to find out the IP of the other person whome your are chatting with or the Ip of the person whome you have got a mail from Its possible .

But only to the Public IP. I mean internet is a collection of Computers and Routers with Public IP's.

And generally a public IP is shared by multiple people using the ISP Service.

If you get a mail from a user by carefully exmaning the email headers you can trace the route for the email. From which IP address has the mail come, and from which server etc.

When you know the IP address you can go to www.dnsstuff.com to find more information about that IP.

Similarly when you are chatting with someone and want to find out from which location he is chatting.

just sent him a big file once he accepts the file a connection would be established between his computer and your computer.

Open command Prompt and type netstat command to list the connection active from your computer.

If you are using only one messenger you can find out the other persons IP address by trail and error method and some basic principles.

If you need any further clarifications please get back to me,

Answer 2

Yes, the headers on each packet give the IP address it's being sent to, otherwise it couldn't be directed to the proper recipient. If it's not an encrypted connection (e.g. SSL) anyone sniffing the traffic can see it.