Week 7: IP, NAT

#Subnetting

Suppose I am setting up 3 networks at home: a main network for laptops, desktop computers, etc.; a guest network for visitors; and an IoT network to isolate my smart devices like my wifi-enabled printer, toaster, and fruit juicer. I have roughly 25 devices that need to be connected to my main network as well as maybe 10 IoT devices. With my guest network, I want to be able to support parties of up to 100 people, each potentially bringing a phone, iPad, laptop, smart watch, and work laptop.

My ISP only provides me with one publicly routable IP address, so I have to use NAT. Each network will be on its own subnet, and the networks must not overlap.

#Subnetting 1

What minimum subnet size do I need for the main network? Remember to take into account reserved addresses within the subnet.

Answer with /N, where N is the number of bits reserved for the network ID.

#Subnetting 2

What minimum subnet size do I need for the IoT network? Remember to take into account reserved addresses within the subnet.

Answer with /N, where N is the number of bits reserved for the network ID.

#Subnetting 3

What minimum subnet size do I need for the guest network? Remember to take into account reserved addresses within the subnet.

Answer with /N, where N is the number of bits reserved for the network ID.

#Subnetting 4

Assume we start our addressing at 192.168.0.0. Since the guest network (/23) is the largest network, it’s easiest to place it first. What is the network address, broadcast address, and first usable IP address for this subnet?

Answer with a list of three IPv4 addresses, like A.B.C.D,E.F.G.H,I.J.K.L where A.B.C.D is the network address, E.F.G.H is the broadcast address, and I.J.K.L is the first usable IP address.

#Subnetting 5

Now, suppose we start the main network (/27) right after the guest network ends. What is the network address, broadcast address, and first usable IP address for this subnet?

Answer with a list of three IPv4 addresses, like A.B.C.D,E.F.G.H,I.J.K.L where A.B.C.D is the network address, E.F.G.H is the broadcast address, and I.J.K.L is the first usable IP address.

#Subnetting 6

Now, suppose we start the IoT network (/28) right after the main network ends. What is the network address, broadcast address, and first usable IP address for this subnet?

Answer with a list of three IPv4 addresses, like A.B.C.D,E.F.G.H,I.J.K.L where A.B.C.D is the network address, E.F.G.H is the broadcast address, and I.J.K.L is the first usable IP address.

#IPv6

#IPv6 1

If a packet is fragmented, how does IPv4 indicate this? How does IPv6 indicate this? Why does IPv6 do it the way it does rather than how IPv4 does?

#IPv6 2

The IPv6 header has a fixed length of 40 bytes, whereas the IPv4 header has variable length (20–60 bytes). What was the primary design goal for simplifying the IPv6 header?

#NAT

You are a network administrator for a startup. You need to configure a NAT box to connect the local office to the Internet.

The LAN uses a private IPv4 range. Server A in the LAN uses address 192.168.1.10. Server B in the LAN uses address 192.168.1.50.

The router’s LAN-side interface has address 192.168.1.1. The router’s WAN-side interface has address 203.0.113.5.

Assume that the router multiplexes the public address using ports starting from 1024 and then increments the port number by one for each new entry.

#NAT 1

Server A accesses an external web server at IP address 93.184.216.34. It uses 40001 for its local source port. What entry is added to the NAT table?

Assume an entry in the NAT table looks like this: A.B.C.D:P,E.F.G.H:Q where A.B.C.D is the LAN-side IP address, P is the LAN-side port, E.F.G.H is the WAN-side IP address, and Q is the WAN-side port.

#NAT 2

Server B accesses an external web server at IP address 103.43.767.5. It uses 50002 for its local source port. What entry is added to the NAT table?

#NAT Traversal

#NAT Traversal 1

First, we will configure a router’s NAT statically. You have a server running locally at 172.16.0.72:8080. Clients know this server’s public address as 54.45.211.98:80.

Enter the activity with nc nat.cs118.org 10001, and get your server connected to the internet!

#NAT Traversal 2

Now, we have a router that uses UPnP. Send a UPnP message from your host machine to the router so that it can receive messages from the internet. The addresses are the same as the first activity.

Enter the activity with nc nat.cs118.org 10002, and get your server connected to the internet!

#NAT Traversal 3

Now, we will use NAT hole-punching to establish a peer-to-peer connection between hosts A and B. You need to learn each host’s IP address and then have them communicate on public port 5000.

Enter the activity with nc nat.cs118.org 10003, and send messages between host A and host B.

#ICMP

#ICMP 1

Which ICMP message is primarily used by traceroute to identify each hop along the path?

Echo
Destination host unreachable
TTL expired/Time exceeded
Fragmentation required

Answer with a number, like 1.

#ICMP 2

Which of the following is NOT diagnostic information that ICMP can provide?

The IP header’s checksum is incorrect
Packet size is too large and fragmentation is needed
A router cannot route a packet to its destination
A packet’s TTL has expired

Answer with a number, like 1.

#ICMP 3

What part of an ICMP ping header is used to associate specific echo requests with echo replies?

Identifier
Sequence number
Type
Application data

Answer with a number, like 1.

#Tunneling

#Tunneling 1

Alice is using a VPN to tunnel into UCLA’s network from her home. UCLA’s network is 145.27.0.0/16, and its gateway/VPN server is at 145.27.0.1. Alice’s public IP address is 61.126.152.98. Inside UCLA’s network, Alice has obtained an IP address of 145.27.15.99. When connected to the VPN, Alice wants to send a request to 145.27.15.100.

NOTE: UCLA’s network does not use NAT in this question.

Alice’s router sends IP packets encapsulated in IP packets. What are the source and destination addresses in both headers? Answer in the format inner_source,inner_destination,outer_source,outer_destination, where all 4 fields are IPv4 addresses (without the slash/CIDR notation).

#Tunneling 2

There are two private networks (A:192.168.3.0/24 and B:192.168.4.0/24) that are connected using a tunnel. The gateway for network A has a public address of 137.1.23.31 and the gateway for network B has a public address of 137.1.24.31. Host 192.168.3.5 in network A sends a message to host 192.168.4.5 in network B. When this message is on the global internet, it has an inner IP header encapsulated with an outer IP header, as before.

What are the source and destination addresses in both headers? Answer in the format inner_source,inner_destination,outer_source,outer_destination, where all 4 fields are IPv4 addresses (without the slash/CIDR notation).