Sub Domains Legitmacy

[Chris]

I Have serverl sub domains off my main domain, example:
sub.mydomain.com

I need to ftp to these sites so I configure the zone file for ftp as follows:

ftp.sub       IN A  1.2.3.4

so when I ftp i use  http://ftp.sub.mydomin.com
I get a message saying "cannot locate server, yet I can ping 
ftp.sub.mydoamin.com I am lost....Can anyone add any insight to this 
questions.. I do appreciate it..

Chris