You are probably aware that all IP addresses on a network must be unique. This requirement holds for networks cards, routers, access points, and so on. If the network device is on the LAN, its IP address must be unique on the LAN. If the device is on the internet, the address must be unique on the internet. In some cases, you will get a warning informing you that there are duplicate IP addresses, but not always.

The use of DHCP, in which assignment of IP addresses is managed by the DHCP server, and IPv6, which greatly increases the address space, both cut down on the possibility of duplicate IP addresses. Duplicate addresses are most likely on IPv4 addresses in which the addresses are statically assigned, so if you are dealing with such a network, consider the possibility.