The 11-B1 is an Intercontinental Ballistic Missile in use by the UNSC during the Human-Covenant War, used for long-range high-yield nuclear strikes against ground-based targets. It is housed in missile silos, reinforced against military strikes and normally staffed with a UNSCDF security detail.
Two 11-B1 ICBM's can be seen flying overhead on Standoff, often two at a time.