This answer will explain how you can point a subdomain name to an IP Address.
Please Note: It will take between 24 - 48 hours for the new DNS records to become active.
Firstly, you need to ensure your domain is pointed to our nameservers:
If the domain is not pointed to the 123-reg nameservers, you will be unable to manage them from the 123-reg control panel.
For more information on changing your nameservers please view the following article:
How do I change the nameservers for my domain name?
To point your subdomain to your host's IP address you will need to do the following:
In the Domain names section, select the relevant domain name in the drop-down list and click on the Manage button.
Scroll down to the Advanced Domain Settings section and click on the Manage DNS (A, MX, CNAME, TXT) link.
From the Advanced DNS tab.
Select A from the Type drop down list in the Add new entry section.
In the Hostname field enter your chosen subdomain i.e mail.
In the Destination IPv4 address field enter the IP address the subdomain will point to.
Click on the Add new entry button to save the new record.