Datacom & Networking S1 — Cheatsheet
Summary of the 2024–2025 course material
Table of Contents
- Network Models and Components
- Delay and Throughput
- Physical Layer and Cabling
- Data Link Layer and Ethernet
- IPv4 and Subnetting
- IPv6
- Transport Layer
- Application Layer
- Troubleshooting
Network Models and Components
| OSI layer | Function | Examples |
|---|---|---|
| 7 Application | network services for applications | HTTP, DNS, DHCP |
| 6 Presentation | format, compression, encryption | TLS, UTF-8 |
| 5 Session | managing sessions | dialogs/checkpoints |
| 4 Transport | end-to-end, ports | TCP, UDP |
| 3 Network | logical addressing and routing | IPv4, IPv6, ICMP |
| 2 Data Link | frames, MAC, local delivery | Ethernet, Wi-Fi, switch |
| 1 Physical | bits and signals | copper, fiber, radio |
Encapsulation: data → segment/datagram → packet → frame → bits; the receiver decapsulates in reverse.
| Component | Primary task | Domains |
|---|---|---|
| Hub | repeats bits to all ports | one collision and broadcast domain |
| Switch | switches frames on MAC address | collision domain per port; broadcast domain per VLAN |
| Router | forwards packets between IP networks | separates broadcast domains |
| Access point | connects wireless clients to the LAN | shared radio medium |
Delay and Throughput
Total delay consists of processing, queueing, transmission and propagation delay.
transmission delay = packet size (bit) / transmission rate (bit/s)
propagation delay = distance / propagation speed
- Bandwidth: theoretical capacity of a link.
- Throughput: actual data rate achieved.
- Goodput: useful application data per second, excluding headers/retransmissions.
- Latency/RTT: one-way or round-trip time.
- Jitter: variation in delay.
The slowest link is the bottleneck. A fast local link doesn't compensate for a slow uplink, congestion or server delay.
Physical Layer and Cabling
| Medium | Properties |
|---|---|
| UTP | cheap, simple; sensitive to electromagnetic interference |
| STP | extra shielding; correct grounding required |
| Multimode fiber | shorter distances, larger core |
| Single-mode fiber | long distances, small core, laser |
Twisted pair
- RJ45/8P8C with T568A or T568B.
- Straight-through cable: same standard on both ends.
- Crossover: A on one end, B on the other; modern auto-MDI/MDIX often makes this unnecessary.
- Respect bend radius, maximum channel length and untwisted length.
Structured cabling: entrance facility → equipment room → backbone → telecommunications room → horizontal cabling → work area. Label, test and document patch panels and connections.
Link up is not proof of quality
A cable can show a link and still cause many errors or poor speed. Test wiremap, length, attenuation and crosstalk.
Data Link Layer and Ethernet
An Ethernet frame contains a destination MAC, source MAC, EtherType/length, payload and FCS. MAC addresses are 48 bits.
unicast → one recipient
broadcast → everyone in the VLAN (ff:ff:ff:ff:ff:ff)
multicast → a selected group
A switch learns the source MAC address on the incoming port. For an unknown destination it floods within the same VLAN; known unicast goes only to the correct port.
Within IPv4, ARP translates a local IP address into a MAC address. For a destination outside the own subnet, the host looks up the MAC address of the default gateway.
IPv4 and Subnetting
An IPv4 address is 32 bits. The prefix determines the network bits; the rest are host bits.
| Prefix | Mask | Addresses | Classic usable |
|---|---|---|---|
/24 |
255.255.255.0 | 256 | 254 |
/25 |
255.255.255.128 | 128 | 126 |
/26 |
255.255.255.192 | 64 | 62 |
/27 |
255.255.255.224 | 32 | 30 |
/28 |
255.255.255.240 | 16 | 14 |
/29 |
255.255.255.248 | 8 | 6 |
/30 |
255.255.255.252 | 4 | 2 |
Subnetting — fixed method
- Determine the required hosts or number of subnets.
- Choose enough host bits:
2^h - 2 ≥ hosts. - The prefix is
32 - h. - Determine the block size:
256 - relevant mask octet. - Note the network, first host, last host and broadcast address.
For 192.168.10.70/26: block size 64 → network .64, hosts .65–.126, broadcast .127.
Private addresses
| Range | Prefix |
|---|---|
| 10.0.0.0–10.255.255.255 | /8 |
| 172.16.0.0–172.31.255.255 | /12 |
| 192.168.0.0–192.168.255.255 | /16 |
APIPA/link-local: 169.254.0.0/16, usually a sign that DHCP failed.
IPv6
IPv6 is 128 bits and uses hexadecimal groups. Shorten one contiguous run of zero groups with :: and drop leading zeros per group.
| Type | Prefix/example |
|---|---|
| Global unicast | generally 2000::/3 |
| Link-local | fe80::/10 |
| Unique local | fc00::/7 |
| Multicast | ff00::/8 |
| Loopback | ::1 |
There is no broadcast in IPv6; Neighbor Discovery uses ICMPv6 multicast. A LAN normally gets a /64. SLAAC uses router advertisements; DHCPv6 can provide additional or stateful configuration.
Transport Layer
| TCP | UDP |
|---|---|
| connection-oriented | connectionless |
| reliable, ordered, acknowledgements | best effort |
| flow and congestion control | low overhead/latency |
| web, mail, file transfer | DNS, streaming, voice, DHCP |
TCP typically starts with SYN → SYN-ACK → ACK. Ports identify applications; a socket combines protocol, IP and port.
Well-known ports: SSH 22, DNS 53, HTTP 80, HTTPS 443, DHCP server/client 67/68.
Application Layer
DHCP — DORA
DHCP hands out IP, prefix/mask, gateway, DNS and lease. A relay forwards broadcasts to a DHCP server outside the local subnet.
DNS
DNS translates names into data. Key records: A, AAAA, CNAME, MX, NS, PTR, TXT.
HTTP(S)
HTTP uses requests/responses; HTTPS adds TLS for confidentiality, integrity and server authentication.
Troubleshooting
Work from the bottom up:
- Power, cable, link LED, Wi-Fi signal.
- Is the interface up with the correct IP/prefix?
- Is the own stack and default gateway reachable?
- Is there a route to an external IP?
- Does DNS resolve?
- The specific TCP/UDP port and application?
Separate DNS from connectivity
If an IP address works but a hostname doesn't, investigate DNS. If the IP address doesn't work either, look lower in the stack.