 TTL for NXDOMAIN (non-existent domain) responses is set from the minimum of the MINIMUM field of the SOA record and the TTL of the SOA itself, and indicates how long a resolver may cache the negative answer.Shorter TTLs can cause heavier loads on an authoritative name server, but can be useful when changing the address of critical services like web servers or MX records, and therefore are often lowered by the DNS administrator prior to a service being moved, in order to reduce possible disruptions.The units used are seconds.