This counter tracks DHCPNAK packets the server has sent, each representing a client's lease request being explicitly refused -- commonly because the client asked to keep an address that's no longer valid for its subnet, already expired, or already assigned elsewhere. A rising Nacks count alongside high DhcpRequests but flat DhcpAcks is a concrete sign clients are struggling to renew or obtain valid leases, which could point to a pool that was recently resized or an address range conflict. For instance, if a client moved to a different VLAN and tries to renew its old address via this server, the server would respond with a Nack rather than an Ack.