Until 1972, Sudan was a single DXCC entity.

However, the Sudanese government were no longer in control of the Southern Sudan area, following the end of the 1972 First Sudanese Civil War. 

This resulted in the original entity of Southern Sudan being added to the DXCC list under a "separate administration" rule.

The political status of Southern Sudan changed again in 1983, when the 21-year long Second Sudanese Civil War started.

The ARRL DXAC and Awards Committee did not vote to delete the original DXCC entity of Southern Sudan until 1998-03-12.
The applicable DXCC rule at the time of this deletion required the retrospective deletion of the entity, back to the start of the war in 1983. However, for reasons of fairness, workload and cost, it was decided to allow QSOs with this entity to count for DXCC up to 1994-12-31.

In January 2011, a referendum of the people of south Sudan voted in favour of independence from the remainder of Sudan. 
On 2011-07-11 the UN General Assembly admitted the Republic of South Sudan as its 193rd member state. 
Following this decision the Republic of South Sudan became a new DXCC entity, effective 2011-07-14. 

On 2011-08-05, the first operation from the Republic of South Sudan, by ST0R in July/August 2011, was approved for DXCC credit by ARRL.

The DXCC mapping is summarised below:

ADIF DXCC No DXCC status Date Prefix
Sudan 466 Current Before 1972-05-07 and from 1995-01-01 ST0-9
Sudan 466 Current From 1972-05-07 until 1994-12-31 ST1-9
Southern Sudan 244 Deleted From 1972-05-07 until 1994-12-31 ST0
The Republic of South Sudan 521 Current From 2011-07-14 Z8