How Custom Domains Work

Instead of using long blog addresses like "thestoryofmyschoolyearsinamerica.blogspot.com" you can use Custom Domains to redirect the above blog to somethng more Web 2.0ish like "www.sasn.com" (Short And Sweet Name). Now your visitors need only type 'sasn.com' to visit your site.

To use Custom Domains you need only two things :

1. Create a Blog on Blogger.com.
2. Buy a Domain Name from a Domain Registrar or from Google.
 

To know what domain names are see Blogger and Wordpress on Same Domain. Buying it from Google costs only 10$ per year (Payment by credit card only!) and comes with a lot of features like domain manager and Google Apps. Buying from other vendors will mean shelling out more for hosting charges (unnecessary since your blog is hosted at blogger) or for domain management. Let us see how Custom Domains work..................
EXTERNAL CUSTOM DOMAIN
To demonstrate this process I bought a domain name "vintip.com" and created a subdomain on it "seo.vintip.com". I requested my domain registrar Bluehost.com to change the CNAME record for 'seo.vintip.com' to point to 'ghs.google.com'. Now this is what happens when a viewer keys in http://www.seo.vintip.com in address bar of web browser :


1. The computer sends the request for seo.vintip.com to my Internet Service Provider (or ISP).
2. My ISP has a Domain Name System server (or DNS). The request is sent to the DNS which has a database of all IP numbers.
3. The CNAME record is looked up and it is directed to Google servers at ghs.google.com.
4. Google servers serve up my blogpage at http://seoblogadsense.blogspot.com. The address continues to show http://www.seo.vintip.com.


My blogspot blog (seoblogadsense.blogspot.com) has been redirected to the Custom domain (seo.vintip.com). You can check by clicking on this original seoblogadsense blogspot address. It will go to http://seo.vintip.com.

CUSTOM DOMAIN THROUGH GOOGLE
If you buy a domain name through Google :
Login at Blogger.com. 

Click Settings link for your blog on Dashboard. 

Click Publishing subtab of Settings tab. 

Click Switch to Custom Domain. 

Type in address and click 'Check availability' button. 

Follow the prompts to buy the domain.

You can choose either Godaddy.com or enom.com to buy it through.

then Google autoconfigures your domain settings. Since Google Apps are involved you have to set the A name record as well as the CNAME records.


To set the CNAME record go to :

https://www.google.com/a/cpanel/YOUR_DOMAIN.COM/DomainSettingsAdvancedDns

Replace 'YOUR_DOMAIN.COM' with the domain name you bought. Sign into your DNS console with the login information provided on this page. When you access Advanced DNS settings, you'll be directed away from Google, and to the domain registration partner that registered your domain name. Create a CNAME record pointing YOUR_DOMAIN.COM to ghs.google.com. If you want a subdomain then first create it and then create a CNAME record for it pointing to ghs.google.com.

The CNAME record points your domain name to another hostname.

Set the A record of your domain as shown below :

mydomain.com. 3600 IN A 64.233.179.121
mydomain.com. 3600 IN A 66.249.81.121
mydomain.com. 3600 IN A 72.14.207.121

As you can see A records link a domain, or subdomain, to an IP address. Now let us see what happens here :
1. The computer sends the request for MYDOMAIN.com to your Internet Service Provider (or ISP).
2. Your ISP has a Domain Name System server (or DNS). The request is sent to the DNS which has a database of all IP numbers.
3. The CNAME record directs it to Google servers at ghs.google.com.
4. The A record direct it to Google Apps servers which serve up the blog page.

CHECK CNAME and A Record for DOMAIN

To check that the CNAME and A Record for your domain has been set correctly go to :

http://www.iptools.com/

Paste in your MYDOMAIN.COM in the "DNS Lookup" Tool to look up DNS record(s) (ex. A, MX, NS). Click the down arrow and select 'CNAME' or 'A' and click GO button. The correct response for CNAME should be :

;; QUESTION SECTION:
;MYDOMAIN.com. IN CNAME

;; ANSWER SECTION:
MYDOMAIN.com. 12654 IN CNAME ghs.google.com.

The correct response for A Record should be :

;; QUESTION SECTION:
;MYDOMAIN.com. IN A

;; ANSWER SECTION:
MYDOMAIN.com. 3600 IN A 72.14.207.121
MYDOMAIN.com. 3600 IN A 64.233.179.121
MYDOMAIN.com. 3600 IN A 66.249.81.121

"Another blog is already hosted at this address" error
If you see the above error you can get it repaired by going to the 'Magical Custom Domain Form' :

https://spreadsheets.google.com/viewform?key=pZHHZdeYKeHitQbvxkvwnCg&

Type in your custom domain (not your blogspot.com address)and click Submit.

Source : http://www.blogdoctor.me/2008/10/how-custom-domains-work.html