National sides across Europe will compete in the new league format that will culminate in a Final Four showpiece event in 2019 for the title of Nations League champion.
UEFA have also confirmed that Nations League games will stand as "competitive" replacements for worldwide friendlies, but not as a replacement for the Euro 2020 qualifying campaign, which will now begin in March 2019 and end in November 2019.
Elsewhere, League A consists of the 12 top-ranked teams: Germany, Portugal, Belgium, Spain, France, England, Switzerland, Italy, Poland, Iceland, Croatia and the Netherlands.
The competition will take place over six matchdays - double-headers during the worldwide breaks already on the football calendar in September, October and November 2018.
The victor of each of the four groups will be drawn to face each other in a Euro 2020 play-off.
The results of the past week's World Cup qualifiers cemented each country's starting position in the Nations League - a biennial four-division tournament which replaces friendly matches from September.
The group winners in Leagues B, C and D will gain promotion and those who finish bottom of the groups in Leagues A, B and C will be relegated.
The January draw will see the Leagues (A, B, C and D) divided into 16 groups of three or four teams, with A being split into four groups of three teams, B the same, C into three groups of four and one of three, and D into four groups of four.
If the group victor has already qualified for Euro 2020, the spot goes to the next best team who haven't and UEFA will simply go down through the Leagues until the mini-tournaments are fully stocked, thus affording both big teams who missed the cut (the Netherlands, as a hypothetical example) and no-hoper minnows alike a potential opportunity to claw their way into Euro 2020.
League C contains Hungary, Romania, Scotland, Slovenia, Greece, Serbia, Albania, Norway, Montenegro, Israel, Bulgaria, Finland, Cyprus, Estonia and Lithuania. However, if a whole league does not have enough teams for a play-off, which is conceivable in League A, then the places will be filled by the best-ranked teams from the league below.
League D: Azerbaijan, Macedonia, Belarus, Georgia, Armenia, Latvia, Faroe Islands, Luxembourg, Kazakhstan, Moldova, Liechtenstein, Malta, Andorra, Kosovo, San Marino, Gibraltar.
Each league will have its own play-off competition with single-leg semi-final and final to decide who gets the final four places.
Teams will be split into one group of three and three groups of four.
Euro 2020 qualification will begin after the group stages of the Nations League concludes and will run throughout 2019, with the play-offs taking place the following Spring.
Wales have missed out on potentially money-spinning games after failing to make it into League A in the new UEFA Nations League format.