Every Portfolio site has a built-in address in the form yourname.myportfolio.com. Connecting a custom domain changes how that address behaves.
Before you connect a custom domain
Your site is served at yourname.myportfolio.com.
After you connect a custom domain
Your custom domain becomes the address your site is served at, and yourname.myportfolio.com permanently redirects to it. Links and bookmarks that use your myportfolio.com address keep working - visitors land on your custom domain.
This also means your myportfolio.com address depends on your custom domain working. If the domain is misconfigured, both addresses appear broken. See My custom domain and myportfolio.com are both down.
Changing your myportfolio.com address
You can change the yourname part of your address, even while a custom domain is connected. You can do this up to five times.
Connecting a custom domain and changing your address behave differently. Connecting a domain leaves your old links working, because your myportfolio.com address redirects to the domain. Changing your address does not: the old myportfolio.com address stops working entirely and is not redirected, so any existing links that use it break. See Change your built-in myportfolio.com address.
Removing a custom domain
If you disconnect your custom domain, your site is served at yourname.myportfolio.com again and the redirect stops.