Finding a domain name ending with worldpay.com or rbsworldpay.com from Dns.GetHostEntry

Finding a domain name ending with worldpay.com or rbsworldpay.com from
Dns.GetHostEntry

Can any one please help me to find domain name ending with worldpay.com or
rbsworldpay.com from
IPHostEntry entry = Dns.GetHostEntry(givenip);
var hostName = entry.HostName.ToLower();
Now i want to make sure hostName ends only with worldpay.com or
rbsworldpay.com and things else i want to throw an exception.
Can any one please help.
Even regular expression check should do?
Thanks