Workstation loses internet

Have a couple of workstations that have a DHCP address and intermittently lose (only) internet connectivity. Their LAN connection remains stable. They can ping the gateway and the firewall, but are unable to ping an external IP address past the firewall. At first I thought it was DNS but they can't ping an external IP address. The outage lasts 2-8 minutes and then it's fine. I've tried searching for duplicate IP addresses on the LAN but don't see any. If I assign them a static IP address, outside the scope of DHCP it seems to resolve the issue. I can also ping their workstation from any other workstation on the network, however, NBTSTAT won't return a host for those affected users.
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JohnBusiness Consultant (Owner)Commented:
Try resetting TCP/IP and DNS Flush. Restore to DHCP (not Static) first.

Open cmd.exe with Run as Administrator
Then  netsh int ip reset c:\resetlog.txt
Also, ipconfig /flushdns followed by net stop dnscache followed by net start dnscache
Then restart the computer

This may fix the connection issues.
It won't be a DNS issue because he can't even ping out using IP address.

"At first I thought it was DNS but they can't ping an external IP address. "

Can they ping inside when they are unable to ping out?  It could be unstable network connectivity or bad cabling.  We have an issue with a network drop that's randomly inconsistent.

You may also want to do a traceroute to see how far they can ping and also pay attention to the hops and how long for each hop.  It should be fast when internal.
JohnBusiness Consultant (Owner)Commented:
Resetting TCP/IP is a good first step to restoring proper network activity. I flush DNS as a matter of course afterward.

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HELFITAuthor Commented:
I'll try resetting the TCP first. When it's happening, and I try a traceroute to an external IP, it doesn't even get to the first hop, the gateway, but I can ping the gateway, which is odd.
Sounds like the first hop is also the gateway/firewall.  You can ping it but it doesn't mean that it will allow traffic out from those two machines and when you give the machines a different IP address then it's fine.

Do you have a special firewall rule set specifically for the DHCP scope in question?
HELFITAuthor Commented:
The gateway and firewall are two separate devices. When this occurs, the tracert never reaches the gateway, but can ping it. I've had two of the workstations drop connection so did as John suggested and reset the TCP. We'll see if that sticks. I also will be trying to ping the workstation from the gateway if that happens again. This also happened again on a user that I gave a static IP to, so that's not a workaround.
JohnBusiness Consultant (Owner)Commented:
Let us know if the reset worked (stopped dropping internet). Also look at resetting your DHCP server and make sure you have enough addresses to hand out for the number of devices you have.
HELFITAuthor Commented:
There are enough addresses. This happened on another workstation. I've learned that the workstation can ping the gateway but the gateway can't ping the workstation when they lose their internet connection.
JohnBusiness Consultant (Owner)Commented:
What owns DHCP?   Server or Router?
What owns the Gateway?  Router probably.

You might want to see if there is a firmware update for your router, update it and reset it. You might want to reset to factory settings and set it up again.
HELFITAuthor Commented:
DHCP is runs on a windows server within the domain. This happens on workstations that have a static ip address as well. Router is a Cisco switch. factory settings is not an option.
HELFITAuthor Commented:
What is confusing me now is that the workstation can ping the gateway, but the gateway can't ping the workstation.
JohnBusiness Consultant (Owner)Commented:
Something appears to have gone screwy with the Cisco router.

I also suggest you find a break in time and restart the Server. Caution: If you have not restarted the Server recently, it will be 9 months pregnant with updates. It just may need to apply updates.
Benjamin Van DitmarsSr Network EngineerCommented:
if the client can ping the firewall and the firewall cant ping the client back. i hope you shutdown the windows firewall. then it looks like an arp problem. can you look at the arp table in the firewall and find the reservations for the clients that cant access the internet. and see if the showed mac-address is correct. if not. flush the arp cache of the router
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