Iohannes is known only through the inscriptions that recorded him as the Consul of year 456, when he was chosen by the Eastern court to hold this office together with Varanes; in the West, however, he was not recognised, as the consulate was held by Emperor Avitus.
Jones, Arnold Hugh Martin, John Robert Martindale, John Morris, "Ioannes 21", The Prosopography of the Later Roman Empire, volume 2, Cambridge University Press, 1980, ISBN0-521-20159-4, p.599.