AS5972 AS5972

A US Department of Defense network from the block DNIC-ASBLK-05800-06055,

registered to the DoD Network Information Center (DNIC). Its address space is

used by the Military Health System — reverse DNS across the AS resolves

overwhelmingly under health.mil and mhsi.med.osd.mil.

What it's used for

• Networking for DoD military medicine: military treatment facilities, hospitals and clinics, and the supporting health-IT infrastructure run under the Defense Health Agency.
• It is one of the largest ASNs in the DoD block by address count, announcing well over a thousand prefixes and roughly 1.9 million IPv4 addresses.

Notable

• A government / DoD network, not a commercial ISP or host.
• The inventory is unusually diverse for a single AS: it aggregates space from the DoD-held 214.0.0.0/8 alongside a long list of legacy /16s (138.x, 139.x, 144.146.x, 160.x, 164.250.x, 199.x), all consistently named for health-system use.
• Reverse DNS names identify individual sites (for example Fort Sam Houston and Beale AFB), which is a good reminder that even DoD ranges leak topology through PTR records.

Interesting

• Health care is why a "military" AS is one of the biggest in the DoD block: hospitals and clinics need far more endpoint addressing than a typical operational network, so the medical enterprise ends up with an outsized share of DoD address space.