Overview
A Unique IP address is an IP that's assigned specifically to your domain or subdomain, instead of the shared IP most hosting accounts use by default. Most sites never need one, but certain applications require a Unique IP to function correctly.
Background
A Unique IP is an IP address that is specific to your domain. When you add hosting to a domain/subdomain, it’s assigned an IP on the server, which is shared with other sites. This shared IP address is most often the only IP you’ll ever need. However, certain applications may require adding a Unique IP to your site.
One IP per domain/subdomain
When you purchase a Unique IP, it is only applied to a single domain or subdomain. If you would like a Unique IP address for your domain (example.com) and subdomain (blog.example.com), you must purchase a separate Unique IP for each domain.
Static IP Addresses
DreamHost is unable to offer static IP addresses. This means that your site's IP (including Unique IPs) is subject to change if necessary. DreamHost makes no guarantees that an IP address will never change. However, in this rare case, you will be notified via email before an IP change occurs.
When do I need a Unique IP address?
You don't need a Unique IP address in most cases; however, some situations may require it. If necessary, you can then add a Unique IP to any Fully Hosted domain/subdomain on the Manage Websites page.
You cannot add a Unique IP address to dreamhosters.com subdomains or DreamPress sites.
How do I allow older browsers to visit my site?
Secure hosting can be used without a Unique IP, but some older browsers that do not support Server Name Indication will display a certificate warning when viewing your site.
While very unlikely, if you expect a significant number of visitors to your secure site that use these older browsers, you will need a Unique IP.
How do I access a site directly?
If you attempt to access your site through the shared server IP, the DreamHost Site Not Found displays.
A Unique IP is necessary if you are interested in accessing the site directly with an IP address. This is also useful when the domain is not yet using DreamHost's nameservers/DNS records.
How do I bypass restrictive firewalls?
A Unique IP is necessary if another domain hosted on the same shared IP as yours is blocked by certain restrictive firewalls (e.g., Great Firewall). Adding a Unique IP address to the site may help it bypass filtering (assuming the filter is based on the shared IP). This approach may also be of interest if you want to be sure your site is not associated with other sites, regardless of the presence of filters. An example of this can be found in the China Firewall article.
How do I add a Unique IP to my email server?
Unique IPs are only for websites. Adding a Unique IP will not affect your mail service since your mail is on a completely different server. There is currently no way to adjust the IP address of a mail server.
Does a Unique IP improve my page rank?
Adding a Unique IP will not improve your site's search engine ranking.