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Byron, PC Support Tech
Category: Networking
Satisfied Customers: 2735
Experience:  CCNA, A+, MCSE 11 years at a large law firm.
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I have a cable modem from my ISP. When I connect my laptop

Customer Question

I have a cable modem from my ISP. When I connect my laptop to the modem via ethernet cable and run an ipconfig or do a "whats my ip" search on a browser I notice the modem gets a 190.x.x.x. public ip address from the ISP. However, if I take out the ethernet cable and plug it into my wireless router so the wireless router is now broadcasting the internet signal from the modem and I do a "whats my ip" on my browser I get a public ip of 181.x.x.x. Can you tell me what's happening?
Submitted: 1 year ago.
Category: Networking
Expert:  Byron replied 1 year ago.

Hi. My name is***** you for your question and the opportunity to assist.

When you take the cable out of your laptop and plug it in to the router, the router requests

an IP address using DHCP (dynamic host configuration protocol)

The IP address you got using your laptop is tied to the MAC address of your network adapter.

When the cable gets plugged in to a different network adapter (your router internet port and it's MAC address) it gets a different

IP address.

Customer: replied 1 year ago.
Doesnt make sense. Explanation is insufficient. Why would the router request ANOTHER PUBLIC IP address from the ISP if one was already assigned to the modem? Also, are you telling me if I turn off the DHCP service on the router then the router and every other computer that connects to the router wirelessly will receive the public ip assigned to the modem?
Customer: replied 1 year ago.
Posted by JustAnswer at customer's request) Hello. I would like to request the following Expert Service(s) from you: Live Phone Call. Let me know if you need more information, or send me the service offer(s) so we can proceed.
Expert:  Byron replied 1 year ago.

No. DHCP on the router only hands out IP addresses for the local LAN.

When you set up the router you specified your type of internet connection. In the case of cable companies, they use DHCP whereas a DSL connection utilizes PPPoE.

If you look at the Properties for the network adapter in your laptop you will see that it's set to automatically obtain an IP address (that's DHCP)

Expert:  Byron replied 1 year ago.

If you were to connect your laptop to the router and go into the router configuration pages and clone the MAC address of the laptop into the router the IP address would stay the same when you switch the cables.

In that situation, no matter if the laptop were directly connect to the modem or connected to the router first. The cable modem would see the same MAC address.

Expert:  Byron replied 1 year ago.

There is no public IP address assigned to the modem. It passes the public IP address to the device attached to it.

Any time a network adapter is connected to a network it makes a DHCP request for an IP address. In the case of a cable customer the request

is sent to the providers router which issues the IP address. IP addresses are leased. Meaning they only last for a certain amount of time. Typically 3 to 7 days for cable companies though they can be set to any length.

They can also be renewed which can result in the issuance of a different IP address.

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