How are you initializing the ethernet shield? Insure you use a valid dns server ip in the setup().
// your network setup stuff
IPAddress ip(192,168,0,14);
IPAddress gateway(192,168,0,1);
IPAddress subnet(255,255,255,0);
// the dns ip must be a valid dns server ip
IPAddress dns(192,168,0,1);
// then in the setup() function
Ethernet.begin(mac,ip,dns,gateway,subnet);