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Discussion 9

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CS 168, Summer 2025 @ UC Berkeley

Slides credit: Sylvia Ratnasamy, Rob Shakir, Peyrin Kao, Iuniana Oprescu

DHCP, ARP, NAT 🚚

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Logistics

Project 3A: Transport

Deadline: Tuesday, July 29th

Project 3B: Transport

Deadline: Tuesday, August 5th

Final exam is on Wednesday, August 13th, 3-6PM

The final accommodations form is open. If you need any exam accommodations (DSP, online, left handed desk, etc.), please fill this form out by Wednesday, August 9th.
For the midterm, a lot of requests came in for changes after the deadline, and while we were lenient, we may not be for the final, so please make sure to fill this out.

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Addressing

DHCP (Application)
ARP (Datalink)
NAT

Transport

Network

Datalink

Physical

Application

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Addressing

DHCP (Application)
ARP (Datalink)
NAT

Transport

Network

Datalink

Physical

Application

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DHCP - Dynamic Host Configuration Protocol

Enables a host to learn about its….

Own IP Address
Network Mask
First-hop router’s IP Address
DNS Server(s) IP Address(es)

INTERNET

?

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DHCP Discovery

DHCP server(s) located on same local network as host
Host initially broadcasts discover message
“Can anyone give me a configuration?”

Local Network

DHCP Server

IP Address: ???

Subnet Mask: ???

First-hop Router IP Address: ???

DNS IP Address: ???

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DHCP Offer

DHCP server(s) responds by broadcasting offer message
Message includes assigned IP address, network mask, first-hop router address, DNS server addresses

Local Network

DHCP Server

IP Address: 212.0.0.0 (tentative)

Subnet Mask: /8 (tentative)

First-hop Router IP Address: 212.2.3.0 (tentative)

DNS IP Address: 212.4.7.8 (tentative)

IP Address: ???

Subnet Mask: ???

First-hop Router IP Address: ???

DNS IP Address: ???

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DHCP Request

Host responds by broadcasting request message
This message identifies which offer was accepted (helps when there are multiple local DHCP servers)

Local Network

DHCP Server

IP Address: 212.0.0.0 (tentative)

Subnet Mask: /8 (tentative)

First-hop Router IP Address: 212.2.3.0 (tentative)

DNS IP Address: 212.4.7.8 (tentative)

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DHCP Acknowledgement

Host responds by broadcasting request message
This message identifies which offer was accepted (helps when there are multiple local DHCP servers)

Local Network

IP Address: 212.0.0.0 (tentative)

Subnet Mask: /8 (tentative)

First-hop Router IP Address: 212.2.3.0 (tentative)

DNS IP Address: 212.4.7.8 (tentative)

IP Address: 212.0.0.0 (confirmed!)

Subnet Mask: /8 (confirmed!)

First-hop Router IP Address: 212.2.3.0 (confirmed!)

DNS IP Address: 212.4.7.8 (confirmed!)

DHCP Server

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DHCP Summary

Client Discover: The client broadcasts a request for a configuration.
DHCP Offer: One or more DHCP servers respond with a configuration offer.�Offer includes subnet mask, router's IP address, DNS resolver, and IP for client.
Client Request: The client broadcasts which configuration it has chosen.�If multiple DHCP servers made offers, the ones that were not chosen discard their offer.
DHCP Acknowledgement: The chosen server confirms that its configuration has been given to the client.

source: lecture 18 slide 20

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A Couple Questions…

1. Dealing with failures?

Hosts have a lease periods for their IP addresses
Hosts must refresh before lease period ends

2. Why do we need the first-hop router’s IP address and the subnet mask?

Answered when we talk about ARP.

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Addressing

DHCP (Application)
ARP (Datalink)
NAT

Transport

Network

Datalink

Physical

Application

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ARP (Overview)

When host sends packet, specify dest Ethernet (MAC) address so packet can traverse local networks
Each host has ARP table, which maps IP to Ethernet
If mapping unknown, ask (solicit) local network by broadcasting “Who has IP address x?”
Host with IP x responds “My Ethernet address is y”

Local Network

Dest Host

IP: a.b.c.d

Ethernet address: 40:11:11:11:11:11

IP Addr.	Ethernet Addr.
a.b.c.d	40:11:11:11:11:11
a.b.c.a	50:37:11:11:11:11

ARP Table

As packet get passed down the stack, it picks up more headers

Layer 3 fills in the IP addresses
Then, Layer 2 needs to fill in the MAC addresses
How do we know what the destination MAC address is? → ARP!

ARP translates Layer 3 IP addresses to Layer 2 MAC addresses.

Steps of the protocol — start with host knowing destination IP but not MAC address:

Host checks cache to see if they already knows the destination MAC address
If destination MAC address is not in the cache, device broadcasts: "What is the MAC address of 1.2.3.4?"
Destination responds by unicasting to host: "My IP is 1.2.3.4 and my MAC address is ca:fe:f0:0d:be:ef."
Everyone else does nothing
Host caches the result

Essentially we need ARP so we can unicast the packet to the right MAC address

could theoretically broadcast: put FF:FF:FF:FF:FF:FF as destination MAC
but now, everybody else has to process this packet
need extra bandwidth to send the packet to everyone on local network

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ARP (Within local network)

Destination is in the same local network

Use ARP table to lookup Ethernet address of dest
Specify Ethernet address when sending packet

Local Network

Dest Host

IP: a.b.c.d

Ethernet address: 40:11:11:11:11:11

IP Addr.	Ethernet Addr.
a.b.c.d	40:11:11:11:11:11
a.b.c.a	50:37:11:11:11:11

ARP Table

Src Host

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ARP (Across local networks)

Destination is NOT in the same local network

How can we tell?

Use subnet mask to check destination network address

Local Network 1

Dest Host

IP: a.b.c.d

Ethernet address: 40:11:11:11:11:11

Src Host

Local Network 1

Src’s First Hop Router

IP: d.c.b.a

Ethernet address: 50:11:11:11:11:11

IP Addr.	Ethernet Addr.
d.c.b.a	50:11:11:11:11:11
d.c.b.c	60:37:11:11:11:11

ARP Table

Router 2

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ARP (Across local networks)

Use ARP table to lookup Ethernet address of first-hop-router (which is in same local network)

We know router’s IP address through DHCP!

Specify first-hop router’s Ethernet address in packet and send packet

Local Network 1

Dest Host

IP: a.b.c.d

Ethernet address: 40:11:11:11:11:11

Src Host

Local Network 1

Src’s First Hop Router

IP: d.c.b.a

Ethernet address: 50:11:11:11:11:11

IP Addr.	Ethernet Addr.
d.c.b.a	50:11:11:11:11:11
d.c.b.c	60:37:11:11:11:11

ARP Table

Router 2

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ARP (Across local networks)

First-hop router will route packet to router 2 using dest IP address

Dest IP address is a.b.c.d. in this example

Local Network 1

Dest Host

IP: a.b.c.d

Ethernet address: 40:11:11:11:11:11

Src Host

Local Network 1

Src’s First Hop Router

IP: d.c.b.a

Ethernet address: 50:11:11:11:11:11

IP Addr.	Ethernet Addr.
d.c.b.a	50:11:11:11:11:11
d.c.b.c	60:37:11:11:11:11

ARP Table

Router 2

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ARP (Across local networks)

Router 2 will use its ARP table to set packet’s dest Ethernet address to actual dest host’s Ethernet address:

Router 2 then sends packet to dest host

Local Network 1

Dest Host

IP: a.b.c.d

Ethernet address: 40:11:11:11:11:11

Src Host

Local Network 1

Src’s First Hop Router

IP: d.c.b.a

Ethernet address: 50:11:11:11:11:11

IP Addr.	Ethernet Addr.
d.c.b.a	50:11:11:11:11:11
d.c.b.c	60:37:11:11:11:11

ARP Table

IP Addr.	Ethernet Addr.
a.b.c.d	40:11:11:11:11:11
a.b.c.c	70:33:33:33:11:11

ARP Table

Router 2

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Putting everything together

Laptop

DHCP

server

DNS

Server

Gateway

Router

google

Server

DHCP: get IP_laptop

UDP: port 67, 68

IP: broadcast

Ethernet: ..

DNS: resolve google.com

UDP: port 53

IP: IP_{DNS_server}

Ethernet,ARP: ..

IP: IP_laptop ↔ IP_google

Ethernet, ARP: ..

The realm of routing protocols..

OSPF, BGP..

HTTP: GET “cat”

IP, NAT: IP_GW ↔ IP_google

Ethernet: ..

..in the Datacenter!

HTTP: OK “=^.^=”

TCP

*Gateway router: the first- and last-hop router of the laptop, e.g., a router at home

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Addressing

DHCP (Application)
ARP (Datalink)
NAT

Transport

Network

Datalink

Physical

Application

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Network Address Translation (NAT)

Only have 2^32 IP addresses
Goal: use single public IP address to represent many hosts
Have a gateway to convert private to public when going in and convert public to private when going out

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Network Address Translation (NAT)

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Network Address Translation (NAT)

Use logical ports to resolve ambiguities
If A and B choose same ports, router notes down fake port

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Worksheet

Multiple Choice
Host-to-Host
Network Address Translation

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Question 1: Multiple Choice

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Question 1: Multiple Choice

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Question 1: Multiple Choice

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Worksheet

Multiple Choice
Host-to-Host
Network Address Translation

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Question 2: Host-to-Host

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Question 2: Host-to-Host

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Question 2: Host-to-Host

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Question 2: Host-to-Host

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Question 2: Host-to-Host

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Worksheet

Multiple Choice
Host-to-Host
Network Address Translation

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Question 3: Network Address Translation

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Questions?

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