If DHCP is leasing IP addresses for RAS, something on your network must
be set up to receive remote connections. The default action for a remote
access server is to lease IP addresses from DHCP. It doesn't have to be a
server. Workstations can also be configured to provide remote access, and
any workstation configured this way will lease an extra two IP addresses
from DHCP (one for the client and one for its end of the connection). If you
are losing 10 or more addresses at a time it must be a server set up as a
remote access server somewhere.