3com vs netgear

We run Windows TSE 4.0 and Citrix Metaframe 1.8 with WYSE terminals (2300se). THe connection works with a terminal-3comhub-network. However, if we try to go from terminal – netgearhub – network, the connection is dropped when the terminal tries tocommunicate with the dhcp server. If I enable DNS and add the host and IP of the dns server ( also the dhcp server) on the network configuration option on the terminal setup, it works. WHY?