NAVIS IMPERIALIS
While the Astra Militarum (Imperial Guard) represents the Imperium's ground forces, the Imperial Navy is responsible for the fleets of starships that maintain order between the stars and planets in the Imperium, for all space and air support provided to the infantry of the Astra Militarum through its Aeronautica Imperialis branch, and for transporting those Guardsmen across the galaxy to the Imperium's myriad warzones. The battlefleets of the Imperial Navy are constantly engaging threats both inside and outside the Imperium's borders.
In ancient times, during the Great Crusade and Horus Heresy eras, the Imperial Navy was not an independent armed force of its own but was considered a part of the greater Imperial Army and was known as the Imperialis Armada.Its vessels were dedicated to the defence of Imperial space and strategic Warp routes. This was opposed to the fleets of warships assigned to the service of the Legiones Astartes, who were used for the planetary assaults that occupied the bulk of the Great Crusade.
After the horrors of the Horus Heresy, during the Imperial Reformation overseen by the Primarch Roboute Guilliman, the Imperial Army's fleet component was reestablished as a a separate and rival service. This was done to weaken the Imperial armed forces in case of the outbreak of a future rebellion against the Emperor's rule. The Imperium is divided into five fleet zones known as the Segmentae Majoris. Every starship of the Imperial Navy is assigned to one of these Segmentae, and falls under the command of the respective Lord High Admiral who commands all the Imperial Navy assets of that Segmentum.
The naval assets of an entire Segmentum are named after that division of the Imperium; i.e. all of the Imperial fleet assets in the Segmentum Solar are known as the Battlefleet Solar, all the assets in the Ultima Segmentum are the Battlefleet Ultima, and so on. There are five Lord High Admirals, one controlling the assembled Battlefleet of each Segmentum. Although all 5 High Admirals are ranked equally, command of the Segmentum Solar is considered the oldest and therefore the most prestigious and senior posting. It is not uncommon for one of these Lord High Admirals to sit on the Senatorum Imperialis as a ruling High Lord of Terra.
The Imperium is divided into five fleet zones known as the Segmentae Majoris. Every starship of the Imperial Navy is assigned to one of these Segmentae, and falls under the command of the respective Lord High Admiral who commands all the Imperial Navy assets of that Segmentum. The naval assets of an entire Segmentum are named after that division of the Imperium; i.e. all of the Imperial fleet assets in the Segmentum Solar are known as the Battlefleet Solar, all the assets in the Ultima Segmentum are the Battlefleet Ultima, and so on.
The fleets of the Imperial Navy are made up of multiple vessels that can be divided into three broad size categories: Battleships and Grand Cruisers, Cruisers, and Escorts. The actual size of starships in the Warhammer 40,000 universe is debatable; however, it is worth noting that the upper size of an Imperial Escort mentioned is 30 kilometers from the Black Library story "Wolf Pack" by Gordon Rennie, though the HDMS Lord Solar Macharius is noted to be just 3 kilometres long. The general consensus is that in the Warhammer 40,000 universe, Imperial Escort vessels are anywhere between 750 metres and 3 kilometres in length, Cruisers are anywhere between 5 and 6 kilometres long and Battleships anywhere between 6 and 8 kilometres.