This layer is the layer in which the various user applications (i.e. web browsing - HTTP; e-mail - SMTP, POP3 and IMAP; File transfers - FTP, TFTP, SMB, KaZaA and SCP) are implemented.
One important application layer service which should be noted is the DNS service, which operates on UDP port 53, and used to translate domain names like www.haifux.org to IP numbers like 132.69.253.254 .
Sometime during the configuration of a connection, one will have to specify the IP of the DNS server of the ISP.