If you use a separate modem and router, then you can change your modem at will without impacting the IP you've been assigned, but changing the router will change the IP, unless your router supports changing the MAC address, and you change it to the MAC of the old one. The LAN IP address is used for accessing your router's web interface to configure settings, while the WAN IP address is provided by your internet service provider to establish external connections. Being a business product, a. IP addresses change because ISPs often assign dynamic IPs, which can change periodically for reasons like network maintenance, reboots, or reconfigurations. Dynamic IPs are useful but can disrupt remote access or gaming setups. To avoid disruptions, you can use Dynamic DNS (DDNS) to track changing. Restarting a router can potentially change your public IP address if it is dynamic, which is often the case with most ISP allocations.
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