Network fundamentals: TCP/IP, ARP, DNS
TCP/IP, addresses, ports, protocols and the journey of a packet.
Level 101 · 19 lessons · 15 practice labs
Lessons
How networks work
- How networks work: layers and a packet's journey: You will understand the layers of a network and the steps a piece of data takes from one computer to another, told as a simple story.
- MAC and IP addresses: You will tell apart MAC and IP addresses and know which one works inside a local network and which one works between networks.
- Static and dynamic IP (how DHCP works): You will understand the difference between fixed and automatically assigned IP addresses and how DHCP hands an address to a device.
- Subnets and CIDR maths: You will work out how subnets and CIDR notation (such as /24) divide a network and set its size.
- Short task: draw up an address plan for a small office
Seeing the connection
- ARP: resolving address to address: You will understand how a device uses ARP to find the MAC address that belongs to an IP address.
- Ethernet frames (a look with tcpdump): You will recognise the parts of an Ethernet frame in tcpdump output and read them on your own lab network.
- Ports and connections (ss, netstat): You will learn what ports are and how ss and netstat show the open connections on a machine.
- Name resolution: hosts, DNS, dig and nslookup: You will see how a domain name becomes an IP address through the hosts file and DNS, and check it yourself with dig and nslookup.
- Routing and the default gateway: You will learn how packets reach other networks through the default gateway and how to read a routing table.
- Short task: prove the ARP, DNS and port steps of one web request
A little further
- Introduction to IPv6: You will recognise what IPv6 addresses look like and how they differ from IPv4 at a basic level.
- VLANs and network segmentation: You will understand why splitting a network into VLANs improves security and keeps damage within a small area.
- Secure access: SSH, TLS and self-signed certificates: You will understand how SSH and TLS encrypt traffic, and why a self-signed certificate raises a warning and what it fails to guarantee.
- Short task: find the IPv6 neighbours and the VLAN tag
Further topics
- NAT and private address ranges: You learn how home and office devices share one public address to reach the internet, and why private addresses are not visible from outside.
- Choosing a network link by need: latency, speed, reliability: You compare wired, wireless and cellular links by latency, speed and outage risk, and pick the right one for each job.
- Short task: identify the address behind NAT and the right link
Wrap-up
- Capstone: map an unknown small network: In a lab you inspect the unknown small network of a fictional company and draw a map showing its devices, addresses and services.
Practice labs
- Who is this machine? Find its own addresses: Find the lab machine’s MAC address, IP address, subnet mask and whether the address came from DHCP, and write them to the answer file.
- Cinnamon Café: which device is static?: Use the lease records and interface settings to decide whether each of the café’s three devices has a static or a DHCP address, and write down the evidence.
- One IP on two devices: the till printer keeps vanishing: Prove from the differing MAC addresses in ARP replies that two devices share one address, find the conflicting device and give it a free address.
- Recognise the gateway in the ARP table: Use the ARP and routing tables to find the gateway’s MAC address and show an Ethernet frame addressed to it in tcpdump.
- Who is asking for whom? Decode the ARP traffic: From a short ARP capture, build a table of which device asked for which address and who answered.
- The server was replaced but ARP still points to the old one: Find the stale or static ARP entry blocking access to a replaced server, clear it and prove with ping and tcpdump that the connection is back.
- Which doors are open on this server?: List the TCP and UDP ports listening on the lab server and the processes that opened them, and write them to the answer file.
- A listening port you don’t recognise: Find which process and which user own an unexpected listening port, and explain which program started it.
- A process talking out: a suspicious outbound connection: Use ss to find the process that keeps opening outbound connections, record the destination address and port, and show the connection stops after you stop the process.
- The hosts file sends you to the wrong address: Find the old /etc/hosts entry that points a name to the wrong address, fix it and show with getent that the right address comes back.
- Two DNS servers, two different answers: Compare with dig why two DNS servers return different addresses for the same name, and write down which one is current and why.
- The name resolves, but from the wrong place: Find and fix several mistakes across nsswitch.conf, hosts and resolv.conf, and prove with dig and getent that an internal name resolves from the right server.
- Kuzey Logistics can’t get out: no gateway: Add the right gateway to a client that has no default route and show it can reach the test server on the outside network.
- Wrong mask: the neighbour answers, the printer doesn’t: Explain which addresses a client with the wrong subnet mask cannot reach and why, fix the mask and prove it with a ping to the printer.
- Who is behind the single address in the log?: Use the connection-tracking table of the lab’s NAT router to find which internal device made the request seen in the outside server’s log.
About this course
TCP/IP, addresses, ports, protocols and the journey of a packet.
What you will learn
- How networks work
- Seeing the connection
- A little further
- Further topics
- Wrap-up
Prerequisites
No prior knowledge is needed; the course starts from zero.
Lab environment
Lessons run in your own Kali Linux terminal in the browser, connected to a private lab network. Nothing to install.
Frequently asked questions
- Is this course right for my level?
- The course is at beginner level; every term is explained where it first appears.
- Is there anything to install?
- No. Every lesson runs in your own lab environment that opens in the browser.
- How do lessons work?
- Each lesson has 8 to 10 steps and a short quiz. Early steps show the command to type, later steps give less help; if two attempts fail, the solution opens.
- Can I try it for free?
- Yes. The first three lessons of every course are free.