Networking Basics
Move from a network overview into addressing, local traffic, routing, transport protocols, and basic troubleshooting.
- Certification
- CompTIA Network+
- Recommended study time
- 7h 30m
- Status
- Not started
Recommended study time
About 7h 30m in total, measured from the material on this page. At your session length of 45 minutes that is 10 sittings.
- Read the lesson20 min
About 2,599 words at a careful technical reading pace.
- Second pass with notes12 min
Re-read the harder parts and write your own notes.
- Work through the examples2h
6 worked examples and 18 practice questions.
- Recall from memory12 min
2 written recall questions.
- Practice decision12 min
One applied decision with feedback.
- Teach it back20 min
Write the topic in your own words.
- Real-world scenario15 min
Read the situation and justify your decision in writing.
- Hands-on practice2h 40m
Labs, commands and configuration until you can do it unaided.
- Spaced review1h 20m
4 short review sessions spread over the following weeks.
Learning objectives
- Recognize IPv4 addresses, subnet masks, default gateways, and private address ranges.
- Explain the different jobs of Ethernet, Wi-Fi, IP, TCP, UDP, and DHCP.
- Apply a layered checklist to a basic connectivity problem.
Start here
About 20 minutes of reading, in 9 short parts.
This lesson turns the conceptual model of networking into concrete configuration: IPv4 addresses, subnet masks, gateways, DHCP, ports, and the diagnostic commands that read them. This is the material you will use on real connectivity tickets every week.
Where you meet it: ipconfig output on a user's laptop, DHCP scopes on a server, port checks against an application, and any 'no internet' ticket.
The lesson, part by part
Open one part at a time. Each part stands on its own, so you can stop and come back.
An IPv4 address is 32 bits, written as four decimal octets such as 192.168.1.40. It has two parts: a network portion and a host portion. The subnet mask defines the split — 255.255.255.0, also written /24, means the first 24 bits identify the network and the remaining 8 bits identify hosts on it.
That split answers one question, and it is the question the operating system asks for every single packet: is this destination on my own network? If yes, deliver it directly over the local link. If no, hand it to the default gateway.
Key ideas
If you remember nothing else from this topic, remember these.
- The subnet mask answers one specific question for every single outgoing packet: is the destination on my own network, and that single answer determines whether a packet is sent directly on the local link or handed to the default gateway.
- An APIPA address in the 169.254.x.x range means the host requested DHCP and received no answer, which is a Layer 2 or DHCP problem specifically, and never a DNS problem, since DNS was never reached at that stage.
- DHCP assigns not just an address but also the subnet mask, default gateway, DNS servers, and a lease time in one four-message exchange known as DORA: Discover, Offer, Request, Acknowledge.
- TCP and UDP solve different problems at the same layer: TCP trades speed for guaranteed ordered delivery through a handshake, while UDP trades reliability for speed, which is exactly why DNS queries and voice traffic use UDP.
- Diagnostic commands should be run in an order that narrows the fault progressively, from the local stack outward to the gateway, then to the internet by IP, then to name resolution, and finally to the specific service port, rather than run randomly.
- A device pinging successfully by IP address but failing to reach the same host by name has a name resolution fault specifically, isolated cleanly from any routing or connectivity problem beneath it.
Diagnosing a user who reaches websites but not the internal file server
A worked example, step by step.
A user reports they can browse the internet fine but cannot open files on the department's shared drive, which points to \\fileserver01\shared.
- 01Check current addressingRun ipconfig /all on the user's machine; it shows a valid address of 10.10.4.55/24, a gateway of 10.10.4.1, and DNS servers pointing to the internal domain controllers, ruling out DHCP or APIPA issues.
- 02Test name resolutionRun nslookup fileserver01; it resolves correctly to 10.10.4.20, so DNS itself is functioning for this name.
- 03Test basic reachabilityRun ping 10.10.4.20; replies come back successfully, confirming the network path to the server is healthy at Layer 3.
- 04Test the specific serviceRun Test-NetConnection fileserver01 -Port 445 to check the SMB file-sharing port specifically; the result shows TcpTestSucceeded : False.
- 05Narrow the service-level faultAsk a colleague on the same subnet to try the same port test against the same server; their result also fails, ruling out a client-specific cause and pointing at the server or a firewall between the segments.
- 06Check the server sideOn fileserver01, confirm the Server service (hosting SMB shares) is running; it shows as Stopped in Services.
- 07Apply the fixStart the Server service and set its startup type back to Automatic, since it appears to have stopped unexpectedly and was not set to restart on its own.
- 08Verify from both machinesRe-run Test-NetConnection fileserver01 -Port 445 from both the original user's machine and the colleague's; both now report TcpTestSucceeded : True, and the shared drive opens normally for the user.
Outcome: The network itself, addressing, routing, and DNS were all functioning correctly the entire time; the actual fault was the SMB file-sharing service stopped on the server, which was found precisely by testing the specific port rather than assuming a general network problem.
Addressing and Diagnostics Facts Worth Memorising
Worth keeping at hand while you work.
- 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16
- RFC 1918 private address ranges, not routable on the public internet
- 169.254.0.0/16
- APIPA range, self-assigned when DHCP fails to respond
- 127.0.0.1
- Loopback address, tests only the local TCP/IP stack
- DORA
- DHCP process: Discover, Offer, Request, Acknowledge
- Port 53
- DNS, used for both queries (usually UDP) and zone transfers (TCP)
- Port 80 / 443
- HTTP and HTTPS respectively
- Port 445
- SMB file and printer sharing
- Port 3389
- RDP, Remote Desktop Protocol
- Port 22
- SSH, also used for SFTP
- ipconfig /release then /renew
- Forces a new DHCP lease on Windows
- Test-NetConnection -Port
- PowerShell command testing whether a specific TCP port is reachable
- tracert / traceroute
- Shows the path and each hop a packet takes toward a destination
Common misunderstandings
What most beginners get wrong here.
A 169.254.x.x address means DNS is broken.
An APIPA address means DHCP was never successfully reached; DNS is not even involved at that point, so the fault sits at Layer 2 or with the DHCP server itself.
If ping to a server succeeds, the application connecting to that server should also work.
Ping only tests ICMP reachability; the actual application relies on a specific TCP or UDP port that could be blocked or the service could be stopped even while ping succeeds.
TCP is used for everything important because it is more reliable than UDP.
UDP is deliberately used for latency-sensitive traffic like DNS queries, DHCP, voice, and video, where the overhead of TCP's handshake and retransmission would hurt more than dropped packets would.
A static IP address is always more reliable than DHCP.
A static address avoids DHCP dependency but introduces its own risk: a device moved to a different subnet with stale static settings will fail silently, and duplicate static addresses cause conflicts DHCP would normally prevent.
The subnet mask is just a formality and any mask will work as long as the IP address is correct.
The subnet mask determines which addresses are considered local versus remote, so an incorrect mask causes a host to misjudge routing decisions and produces intermittent, confusing connectivity failures.
Exam traps
How the question writers try to catch you out.
- A question showing an address of 169.254.x.x is almost always testing recognition of APIPA and a DHCP-related root cause, not a DNS problem.
- Expect port-number memorization questions phrased as scenarios, such as 'a user cannot access a website but the network is confirmed healthy,' pointing to port 80/443 or DNS depending on the exact wording.
- Questions distinguishing ping success from application failure are testing the difference between ICMP reachability and a specific TCP/UDP port or service being available.
- Performance-based questions frequently present raw ipconfig /all output and ask you to identify the misconfiguration directly from the printed values.
- A question describing new devices getting APIPA while existing devices keep working normally is testing recognition of DHCP scope exhaustion specifically.
Check yourself
Answer in your head first, then reveal. This is not scored.
A user's ipconfig /all shows an address of 169.254.32.10. What is the most likely cause and what should you check first?
Why does pinging 8.8.8.8 successfully but nslookup failing point specifically to DNS rather than general connectivity?
What four pieces of information does a DHCP server typically hand out beyond just an IP address?
A user reaches every website fine but cannot connect to an internal file server by name. What two tests would separate a DNS problem from a service-availability problem?
Why is UDP the appropriate choice for DNS queries and voice traffic instead of TCP?
Quick reference
A condensed summary of the lesson above, for revision.
What It Is
An IPv4 configuration normally includes an address, subnet mask, and default gateway. The address identifies an interface; the mask determines which destinations are local; the gateway routes traffic elsewhere. Ethernet or Wi-Fi carries local frames, IP moves packets across networks, and TCP or UDP transports application data. DHCP can supply configuration automatically.
Why It Matters
Most IT services depend on correct addressing and a working path. A valid local link does not prove the gateway or internet works. By testing from the nearest layer outward—interface, address, local peer, gateway, remote address, then application—a technician can narrow the fault instead of changing unrelated settings.
How It Works
- The subnet mask tells a host whether a destination is local.
- Local traffic is delivered over Ethernet or Wi-Fi; remote traffic goes to the default gateway.
- TCP or UDP carries application data while DHCP can supply addressing.
Where You See It
- Endpoint address settings, home routers, enterprise VLANs, server ports, and connectivity tests.
Key Terms
- IPv4 address
- A 32-bit logical address commonly written as four decimal numbers.
- Subnet mask
- A value that separates the network portion of an address from the host portion.
- Default gateway
- The router a device uses for destinations outside its local network.
- TCP
- A connection-oriented transport protocol with ordered, reliable delivery.
- UDP
- A lightweight transport protocol without TCP's delivery guarantees.
- DHCP
- A service that automatically leases network settings to clients.
Examples
- A laptop with a self-assigned 169.254.x.x address may have a working interface but no response from a DHCP server.
- A video call may favor timely UDP traffic, while a file transfer uses TCP to ensure ordered delivery.
Common Problems
- Self-assigned 169.254 address
- Wrong subnet mask or gateway
- DHCP failure
- Blocked application port
How It Fails
- A valid link with no DHCP response leaves the host without usable routed settings.
- A wrong gateway breaks remote access while local peers may still work.
- A firewall can block one service even when ping succeeds.
How to Troubleshoot
- Check physical or wireless link.
- Inspect IP, mask, gateway, and DHCP source.
- Test local stack, local peer, gateway, remote IP, then application port.
Practical Knowledge
- Recognize private IPv4 and APIPA ranges.
- Use ipconfig/ifconfig/ip and ping as evidence, not as automatic fixes.
Exam Coverage
- IPv4 configuration and private ranges
- TCP, UDP, DHCP, Ethernet, and Wi-Fi
- Layered connectivity troubleshooting
Interview Questions
- What does a default gateway do?
- What does a 169.254.x.x address suggest?
Worked examples
Each calculation is shown one step at a time, then you try it yourself before revealing the answer.
Convert binary to decimal
Convert the 8-bit binary number 11000000 to decimal.
- 1. Write the place valuesAn 8-bit number has fixed place values, left to right: 128, 64, 32, 16, 8, 4, 2, 1.
- 2. Line the bits up under the place values128→1, 64→1, 32→0, 16→0, 8→0, 4→0, 2→0, 1→0.
- 3. Keep only the place values above a 1The 128 column and the 64 column hold a 1. Every other column holds 0 and contributes nothing.
- 4. Add them128 + 64 = 192.
Answer: 11000000 = 192. This is why a /24 subnet mask octet of 11000000 reads as 192 in dotted decimal.
Now you try
Convert 10101010 to decimal.
Convert 11111111 to decimal.
Convert 00011100 to decimal.
Convert decimal to binary
Convert the decimal number 172 to 8-bit binary.
- 1. Start at the largest place valueAsk: does 128 fit into 172? Yes. Write a 1 and subtract: 172 − 128 = 44.
- 2. Next column, 6464 does not fit into 44. Write 0. Remainder stays 44.
- 3. Next column, 3232 fits into 44. Write 1. 44 − 32 = 12.
- 4. Continue down16 into 12? No → 0. 8 into 12? Yes → 1, remainder 4. 4 into 4? Yes → 1, remainder 0. 2 → 0. 1 → 0.
- 5. Read the bits in order1, 0, 1, 0, 1, 1, 0, 0.
Answer: 172 = 10101100. Check it by adding the on-bits back: 128 + 32 + 8 + 4 = 172.
Now you try
Convert 200 to binary.
Convert 19 to binary.
Convert 255 to binary.
Convert binary to hexadecimal
Convert 11011110 to hexadecimal (the form used by MAC and IPv6 addresses).
- 1. Split into 4-bit groups11011110 becomes 1101 and 1110.
- 2. Convert each group with place values 8, 4, 2, 11101 = 8 + 4 + 1 = 13. 1110 = 8 + 4 + 2 = 14.
- 3. Replace values over 9 with letters10=A, 11=B, 12=C, 13=D, 14=E, 15=F. So 13 = D and 14 = E.
- 4. Join the digitsD followed by E.
Answer: 11011110 = DE in hex, which is 222 in decimal. Each pair of hex digits is exactly one byte, which is why a MAC address is six hex pairs.
Now you try
Convert 10101111 to hex.
Convert hex 3C to binary.
Convert hex FF to decimal.
Work out a network address and host range
A host is configured as 192.168.10.77 with the mask 255.255.255.192 (/26). What is its network address, broadcast address and usable host range?
- 1. Find the interesting octetThe first three octets of the mask are 255, so only the fourth octet matters. 192 in binary is 11000000, so 2 host-network bits are borrowed.
- 2. Find the block size256 − 192 = 64. Subnets step in blocks of 64: 0, 64, 128, 192.
- 3. Place the host77 falls between 64 and 127, so the host sits in the 192.168.10.64 subnet.
- 4. Name the boundariesNetwork address = 192.168.10.64. Broadcast = one below the next block = 192.168.10.127.
- 5. Usable hostsEverything between the two boundaries: .65 through .126.
Answer: Network 192.168.10.64, broadcast 192.168.10.127, usable range 192.168.10.65–192.168.10.126 (62 usable addresses).
Now you try
Same question for 10.0.0.200 /26.
What mask is /28 in dotted decimal, and what is its block size?
How many usable hosts in a /29?
Count usable hosts from a CIDR prefix
How many usable host addresses does a /22 network provide?
- 1. Count host bitsAn IPv4 address is 32 bits. 32 − 22 = 10 host bits.
- 2. Raise 2 to that power2^10 = 1024 total addresses in the block.
- 3. Remove the two reserved addressesThe lowest address is the network address and the highest is the broadcast address: 1024 − 2.
Answer: 1022 usable host addresses. The general formula is 2^(32 − prefix) − 2.
Now you try
Usable hosts in a /30?
You need 500 hosts on one subnet. What is the smallest prefix?
Total addresses in a /16?
Trace a DNS lookup step by step
A workstation opens www.example.com for the first time. What happens before the first packet reaches the web server?
- 1. Local checksThe resolver checks its own cache, then the hosts file. A stale entry here explains a machine that reaches the wrong server while everyone else is fine.
- 2. Recursive resolverThe query goes to the configured DNS server, usually learned by DHCP. If that server has a cached answer within its TTL, it replies immediately.
- 3. Root and TLDWith no cache, the resolver asks a root server, which refers it to the .com name servers, which refer it to the authoritative servers for example.com.
- 4. Authoritative answerThe authoritative server returns the A record (IPv4) or AAAA record (IPv6) for www.example.com.
- 5. Cache and connectThe answer is cached for its TTL, then the workstation opens a TCP connection to that IP on port 443.
Answer: Name resolution is a chain: cache → hosts file → recursive resolver → root → TLD → authoritative. Test the chain with nslookup or dig; if the IP is correct but the page fails, the fault is below DNS.
Now you try
A site works by IP but not by name. Where is the fault?
Which record type maps a name to an IPv6 address?
Why does a record change take hours to appear everywhere?
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Learning progress
0% across six evidence areas. Reading alone does not change progress.
Prerequisites
Next steps
- 01Inspect your current IPv4 address, subnet mask, default gateway, and DHCP status.
- 02Sketch a layered connectivity checklist from physical link to application.