Template:User shared IP address
Usage
To use include the template with the user name of the other user after the pipe symbol:
{User shared IP address|Username}
Users with this template on their user page are automatically added to Category:Wikipedians with shared IP addresses.
A non-technical explanation
If you share a home or an office with another Wikipedian you probably need to use this template, because in many cases the internet is provided by a single internet service provider (ISP) via a residential gateway. All computers in the premises that have access to the internet are connected to a private network with a local address. To access the internet the residential gateway translates those local addresses into a public routable IP address, therefore to the outside world your computer and all the other computers in the premises appear to share an IP address.
As a result of this shared addresses, it can seem that one person is operating more than one account (which is against policy), when in reality there is more than one person operating their own separate accounts, just from the same network.
NB. There are lots of exceptions to the above which the more computer literate will know about, but those for whom "shared IP address" is just another piece of impenetrable computerese, your home computers will probably be using such a system.