How to initiate a socket connection from server to client? - python-3.x

Question

I have setup a tcp socket between a client and a server, very basic. Client side:
#!/usr/bin/env python3
import socket
# create a socket object
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
# get local machine name
host = "81.88.95.250"
port = 25000
# connection to hostname on the port.
s.connect((host, port))
# Receive no more than 1024 bytes
msg = s.recv(1024)
s.close()
print (msg.decode('ascii'))
server side:
#!/usr/bin/env python3
import socket
# create a socket object
serversocket = socket.socket(
socket.AF_INET, socket.SOCK_STREAM)
# get local machine name
host = socket.gethostname()
port = 25000
# bind to the port
serversocket.bind((host, port))
# queue up to 5 requests
serversocket.listen(5)
while True:
# establish a connection
clientsocket,addr = serversocket.accept()
print("Got a connection from %s" % str(addr))
msg = 'Thank you for connecting'+ "\r\n"
clientsocket.send(msg.encode('ascii'))
clientsocket.close()
My target is to send notification from client to server, and that's easy. The difficult part is that I also need in some circumstances to start the connection from the server and to send a command to the client and this must be executed as soon as it is received, so I cannot setup a periodic "poll". But I'm quite confused on this part, because the client is behind a NAT, not exposed with a public IP.