This answer will explain what a nameserver is, what it does, and what the default nameservers are for 123-reg.
A nameserver is a server which controls the DNS for a domain. It allows you to decide which hosting company controls your webspace and email.
All domain registrars should provide you with the ability to change at least two nameserver entries on your domain name, and hosting companies should offer you at least two nameservers to point to.
The 123-reg.co.uk nameservers are:
One of the advantages of registering your domain name with 123-reg is that you are free to change the nameservers to other hosts whenever you like. However, if you choose not to point to our nameservers, you won't be able to use our web and email systems to configure your domain name, nor will you be able to use our hosting solutions.
Please note: If you change the DNS to another registrar, you will no longer be able configure your DNS changes for that domain within your 123-reg.co.uk control panel.
You can check the current DNS settings for your domain name at www.allwhois.com.