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A12.6 DHCP, NAT and port forwarding

Networks and the web · A level · AQA 7517 4.9.4.7 · about 30 min

What this lesson is about

How a device gets its address, and how a router shares one public address with a whole network.

• DHCP
• NAT
• Port forwarding

Questions 6 marks in all

1. [1 mark]Put the DHCP messages in the order they are sent.

   Number the lines 1 to 4 to put them in the right order.

   1. Offer
   2. Acknowledge
   3. Discover
   4. Request
   Answer:

   Discover
   Offer
   Request
   Acknowledge

   The client discovers a server, the server offers an address, the client requests it, and the server acknowledges the lease.

2. [1 mark]Which settings does a DHCP server typically give a device? Choose all that apply.

   Tick every answer that is true.

   1. AIP address
   2. BSubnet mask
   3. CDefault gateway
   4. DDNS server address
   5. EMAC address
   Answer: A, B, C, D. The MAC address belongs to the device's network interface and is not assigned by DHCP.

3. [1 mark]When a packet leaves a home network through a router using NAT, what does the router change?

   1. AThe private source IP address (and usually the source port) to its public address and a port of its own, recording it in a table
   2. BThe destination IP address to a private address
   3. CThe MAC address of the destination server
   4. DNothing: NAT only affects incoming packets
   Answer: A. The router records the translation so it can change the destination of the reply back to the private socket.

4. [1 mark]Why is NAT used? Choose all that apply.

   Tick every answer that is true.

   1. AMany devices can share one public IPv4 address
   2. BPrivate addresses are hidden from the Internet
   3. CIt speeds up DNS lookups
   4. DIt gives every device a globally unique address
   Answer: A, B. NAT conserves public IPv4 addresses and hides the internal network; unique addresses for every device is what IPv6 offers instead.

5. [1 mark]A student runs a game server at 192.168.1.40 on their home network. What lets friends on the Internet connect to it?

   1. AA port forwarding rule mapping a public port on the router to 192.168.1.40 and the server's port
   2. BGiving the server a DHCP lease
   3. CTurning off the router's DNS
   4. DUsing a thin client
   Answer: A. Without port forwarding, a connection started from outside has no NAT table entry and is dropped.

6. [1 mark]What does this program print?

   table = {}
   next_port = 50000
   for socket in ['192.168.1.5:3000', '192.168.1.6:3000', '192.168.1.5:3000']:
       if socket not in table:
           table[socket] = next_port
           next_port = next_port + 1
       print(socket, '->', table[socket])

   Answer:

   192.168.1.5:3000 -> 50000
   192.168.1.6:3000 -> 50001
   192.168.1.5:3000 -> 50000

   Each new private socket gets the next public port; the repeated socket reuses its existing entry.

The task: get an address

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

send("DISCOVER")
wait(1)
print(messages())

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

def wait_for(word):
    while True:
        wait(0.1)
        for sender, text in messages():
            fields = text.split()
            if fields[0] == word:
                return fields

send("DISCOVER")
offer = wait_for("OFFER")
print("offered", offer[1])
send("REQUEST " + offer[1])
ack = wait_for("ACK")
print("address:", ack[1])
print("gateway:", ack[3])
print("lease:", int(ack[4]) // 3600, "hours")