No. Basta fare ogni tanto il maintain(). 
Non c'è bisogno di un nuovo begin.
int EthernetClass::maintain(){
int rc = DHCP_CHECK_NONE;
if(_dhcp != NULL){
//we have a pointer to dhcp, use it
rc = _dhcp->checkLease();
switch ( rc ){
case DHCP_CHECK_NONE:
//nothing done
break;
case DHCP_CHECK_RENEW_OK:
case DHCP_CHECK_REBIND_OK:
//we might have got a new IP.
W5100.setIPAddress(_dhcp->getLocalIp().raw_address());
W5100.setGatewayIp(_dhcp->getGatewayIp().raw_address());
W5100.setSubnetMask(_dhcp->getSubnetMask().raw_address());
_dnsServerAddress = _dhcp->getDnsServerIp();
break;
default:
//this is actually a error, it will retry though
break;
}
}
return rc;
}