something is calling isc.org from inside my computers on network....?

For few days I've been surprised by a huge amount of connection request from within my network to one of ISC address on port 80.

The address is 204.152.184.139

All requests are block by my firewall.

I've been searching through the net to find some more info regarding this issue.

I'm suspecting something nasty is going on, since there is no alert from our antivirus system ( trendmicro officescan )

I tried to use well-known antimalwares and antispywares programs on a sample computer but found nothing suspicious.

netstat -anb shows :

TCP 172.16.3.232:4021 204.152.184.139:80 SYN_SENT 1444

c:\windows\system32\WS2_32.dll

C:\WINDOWS\system32\WININET.dll

-- unknown component(s) --

[svchost.exe]

Any idea what I'm dealing with??