Member Avatar for kaycam

Strange things are happening with my computers and the Internet
connection. The Internet is working just fine, so is my Mac and PC.

When I logged on to my PC this morning, I got an Internet connection. But when I logged off and plugged the Internet cable into my Mac, the Internet would not work. It
was not until I shut down and unplugged the PC power cord from the power bar that the Internet worked on the Mac.

Now tonight I logged on to my PC, the Internet would not work. I logged off and unplugged the Mac from the power bar, but the Internet still would not work on the PC. So I log back on to my Mac and I get an Internet connection, so I am writing to you from my Mac.

Do any of you happen to have any experience with this scenario? Is it that the Internet connection is not strong enough? Is there a reason why the Internet would be working alternatively like this? I do not wish to connect both computers to the Internet at once. I use the two computers separately with the Internet, either one or the other.

Thanks for your help!
Kaycam

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  • Latest Post Latest Post by airbourne

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Short, practical explanation: the behavior described by matches how many ISP/modem combinations handle the public IP — the modem (or the ISP's DHCP server) effectively "remembers" the MAC address of the last device that established the connection. When a different machine is attached the ISP often won't hand out the same IP immediately, so the new host won't get network access until the lease is updated or the modem is reset. That is why the connection appeared to move back and forth between the PC and the Mac.

A safe short-term method to force the lease to move without power-cycling the modem is to release and renew the DHCP lease on the client. For example:

Windows (Admin CMD): ipconfig /release
Windows (Admin CMD): ipconfig /renew

Linux (terminal): sudo dhclient -r && sudo dhclient

On macOS use System Settings (Network) → select the interface → Advanced → TCP/IP → “Renew DHCP Lease.” These steps request a fresh DHCP assignment and often restore connectivity faster than unplugging hardware.

Longer-term fixes reduce the need to swap or reboot hardware. A NAT router between the modem and machines presents one WAN MAC/IP to the ISP and shares Internet to multiple devices simultaneously; many routers also offer a “MAC clone” option that can mimic an existing PC MAC when an ISP has bound service to one address. For DSL that uses PPPoE, the router must be configured with the ISP username/password. Avoid assigning conflicting static public addresses on two machines — NAT or contacting the ISP for additional public IPs are the correct solutions.

Quick checklist: verify modem/router link lights and correct cables, confirm each client is set to obtain IP automatically when using DHCP, avoid duplicate private IPs on the LAN, and, if problems persist, contact the ISP to confirm whether they bind by MAC or require provisioning when devices change. This supplements the practical notes from , and .

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All 6 Replies

Member Avatar for hughv

Most likely, you need to reboot the modem when you switch connections.
It's perfectly safe to use a router and connect both computers at once, which is what most people do.

Member Avatar for airbourne

Most cable or DSL internet providers will only allow 1 PC online at a time. The first PC to claim the internet connection gets it and another PC cannot use the connection unless that connection expires (which is rare as I understand it). To swap who can get out to the internet, you normally have to reboot the modem.

To share the internet connection you need a router between your computers and the modem. The same rules apply to the router - since the router has to make a connection to the internet, you have to reboot your modem before connecting it to your router.

Member Avatar for naeemaziz

In this case you have to give the manual ip to your MAC thats y the internet will not work without ip it wont work describe me what are u using are u using dsl or what.

Member Avatar for kaycam

Thanks very much. I rebooted the modem and I got it to work.

Kay

Most cable or DSL internet providers will only allow 1 PC online at a time. The first PC to claim the internet connection gets it and another PC cannot use the connection unless that connection expires (which is rare as I understand it). To swap who can get out to the internet, you normally have to reboot the modem.

To share the internet connection you need a router between your computers and the modem. The same rules apply to the router - since the router has to make a connection to the internet, you have to reboot your modem before connecting it to your router.

Member Avatar for kaycam

Thanks very much. I rebooted the modem, and it worked. Thanks. Kay

Member Avatar for airbourne

You are very welcome. :)

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