Unkown ip-address in LAN

We have since more than a week have had a very slow internet connection. I have then looked at the traffic through our gateway (windows server 2008, 2 NIC (Internal & External),Forefront) by using wireshark and discovered that a lot of traffic is originating from a specific source IP-adress (protocol mainly TCP). When I try to ping this IP I do not get any answer. In the LAN DNS there is no entry with that IP. How can the source be an IP which does not exist?
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kaskhedikar_tusharConnect With a Mentor Commented:

Someone is connected the personal device by using WiFi.
You will have to change your WiFi password.
Secure your wireless network by using MAC access control list.
Also check the firewall.

Tushar Kaskhedikar
The end point has a system state firewall that drops ICMP echo (ping) traffic. System state means if the node didn't initiate the traffic, it will not except (meaning drop) the traffic.

The node exists if your LAN scanner shows it as passing traffic. Now, you got to find out what node it is.

Try these:

Ping -a (IP address)
NBTSTAT -c (IP address)
NBTSTAT -a (IP address)

See if this gives you information that was broadcast to the DNS server or Netbios Name server.

If not, try and map a drive to it, or remote manage it by IP. See if you can use admin shares to see what's on the hard drive. ALSO, type in the IP as a web site to go to in a browser. This could be a router or printer or storage device with built in Web page.

Furthermore. If you ask me (this sounds like a rogue router).. Use (on your DC) DHCPloc.exe to see if it's Offering DHCP leases. A rogue router will most often provide DHCP address and it will jack your internet/domain up with performance issues.
AndHofOwnerAuthor Commented:
This was a good answer and I will accept the answer as a solution. However, this was not the problem. We do not have WiFi but your annwer get me thinking in the right direction. The problem is that I'm too stupid. I'm connected to the my office LAN via VPN from home. The IP-address I see is my own VPN-connection. I'm sorry for this.
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