First of all, just in case you are asking What are nameservers? Now that is out of the way.
When creating private nameservers, you will need to register a minimum of two.
To register private nameservers with 123-reg.co.uk, log in to your customer account and then follow this guide.
1. Select the correct domain in the drop-down and click ‘Modify Domain’
2. Click ‘Change Name Servers’ and enter your new name servers in the ‘Name Server 1 and 2’ fields.
Note: These are usually ns1 or dns1
3. Click ‘Change Nameservers’ and enter the IP addresses associated with the new name servers.
4. Click ‘Change Nameservers’ to create the new name servers.
Note: When creating private nameservers, it may take up to 72 hours for these to propagate throughout the internet.