Getting connection refused when trying to connect to a tcp server with python

I've written two programs according to a guide on sockets in python. I'm using a rasbperry pi 3 as a client, and a regular linux ubuntu computer as a server. this is the server software:

import socket
import sys

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

server_address = ('localhost', 15000)
print("starting up on %s port %s" % server_address, file=sys.stderr)
sock.bind(server_address)

# Listen for incoming connections
sock.listen(1)

while True:
    # Wait for a connection
    print("waiting for a connection", file=sys.stderr)
    connection, client_address = sock.accept()

try:
        print("connection from ", client_address, file=sys.stderr)

    # Receive the data in small chunks and retransmit it
    while True:
        data = connection.recv(16)
        print("received %s" % data, file=sys.stderr)
        if data:
            print("sending data back to the client", file=sys.stderr)
            connection.sendall(data)
        else:
            print("no more data from ", client_address, file=sys.stderr)
            break

finally:
        # Clean up the connection
        connection.close()

and this is the client software:

import socket
import sys

# Create a TCP/IP socket
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

# Connect the socket to the port where the server is listening
server_address = ('192.168.18.250', 15000)
print("connecting to %s port %s" % server_address, file=sys.stderr)
sock.connect(server_address)

try:
# Send data
message = "This is the message.  It will be repeated."
print("sending %s" % message, file=sys.stderr)
sock.sendall(message)

# Look for the response
amount_received = 0
amount_expected = len(message)

while amount_received < amount_expected:
    data = sock.recv(16)
    amount_received += len(data)
    print("received %s" % data, file=sys.stderr)

finally:
    print("closing socket")
    sock.close()

this is the output on the server:

peter@GIGAS:~/thermServer$ python3 thermServer.py
starting up on localhost port 15000
waiting for a connection

and this is the output on the raspberry pi:

pi@raspberrypi:~ $ python3 thermClient.py
connecting to 192.168.18.250 port 15000
Traceback (most recent call last):
  File "thermClient.py", line 10, in <module>
    sock.connect(server_address)
ConnectionRefusedError: [Errno 111] Connection refused

I have forwarded the port in my router, but as this is internal traffic that shouldn't matter, did I miss adding something in the server that opens the port properly or do I need to fiddle with something outside of the project in my linux machine?