The Mishna on today’s daf returns us to the question of eiruvei hatzeirot and teaches that a city which once belonged to an individual but became a public city can still have a single eiruv to permit carrying in the city, as it is still perceived as a single hatzer (courtyard). If, on the other hand, a city that once was public became a private city, a single eiruv cannot be made unless an area is left outside the eiruv, so that the rules of eiruv will not be forgotten.
There are a number of explanations of what constitutes a private city. According to Rashi, it means that there are not 600,000 residents, so it is not a reshut ha-rabim, a public domain. (According to Rashi, in order to be considered a reshut ha-rabim the city needs to be similar in number to the encampment of the Israelites in the desert.) The Mishna teaches that even when the number of residents reaches 600,000, the original eiruv will still be valid. The Rashba and Ritva understand that the city was actually owned by a single individual who leased space to others. Even if it is sold later on to a number of people, its original status as a private city gives it a unique status.
The discussion of eiruv hatzeirot in our Mishna appears to be out of place in our chapter, whose focus is eiruvei tehumim. This halakha really belongs in one of the earlier chapters of Massekhet Eiruvin. One suggestion is that this is a segue from the previous Mishna (58b), which taught that we can trust the memory of local people – even a slave or maid-servant – to testify how far the tehum was outside the city. In our case, as well, the power of memory plays a role in establishing the city as either public or private, and that memory carries weight in halakha, even if the situation has changed.