Which address are you referring to? Your ISP gives you 2 addresses when you connect:
1. your modem internet IP address
2. the DNS address
Some ISPs will allow you to have a fixed WAN IP address, but you normally have to pay extra. This would mean that your address on the internet is constant; useful if you are hosting, say, a web page. If this is the address you want unchanged, you must contact your ISP.
If it is the DNS address that you want to keep the same, that is much easier. Part of your server set-up will give you an option to obtain the DNS address automatically or to input the address you want to use. There are very many DNS servers on the web and you can put in the address of any one of them. However, if you know one that works for you, enter it in the box and each time you log on, your server will use that DNS address.
Be aware of 2 issues:
1. If that server goes down, you will lose the DNS service
2. The ISP can change it if he wishes.
In both cases, a new DNS address will be available to servers that are set to automatic so they will continue to work; however, your server will continue to look for its fixed address. You would be advised to give the server an alternate address also.