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Linux Master 2· AssociateWeek 9 · Day 5~19 min read

Day 5 - Week 9 Review: The Network on One Page

📌 Key Takeaways

The OSI 7 layers, bottom to top — Physical, Data Link, Network, Transport, Session, Presentation, Application. A router is Layer 3, a switch is Layer 2, a hub is Layer 1.
CD is for wired (Ethernet), CA is for wireless (Wi-Fi).
An ATM cell is 53 bytes, and FDDI is a dual fiber-optic ring with token passing.
254 · 126 · 62 · 30 · 14 · 6 · 2 — the host-count row for Class C subnets.
hosts is where you write down the answers, resolv.conf is where you write down whom to ask.

The Network on One Page

   OSI 7 layers                     TCP/IP        Device        Unit
   ─────────────────────────────────────────────────────────────────
   7 Application  HTTP·FTP·SMTP·DNS  ┐
   6 Presentation Encryption·compression ├─ Application  Gateway  Data
   5 Session      Connection mgmt    ┘
   4 Transport    TCP · UDP · ports  ── Transport      Gateway     Segment
   3 Network      IP · ICMP · ARP    ── Internet       Router      Packet
   2 Data Link    MAC · Ethernet     ┐
   1 Physical     Cable·signal       ┘─ Network Access Switch/Hub  Frame/Bit
 
   Ports  21 FTP · 22 SSH · 23 TELNET · 25 SMTP · 53 DNS
          80 HTTP · 110 POP3 · 143 IMAP · 443 HTTPS
 
   Switching  Circuit switching (PSTN, dedicated path) ↔ Packet switching (PSDN, cut into pieces)
              Packet switching = Datagram (connectionless) + Virtual circuit (connection-oriented)
 
   Access  CSMA/CD wired · CSMA/CA wireless · Token passing (Token Ring·FDDI)
 
   WAN    X.25 → Frame Relay → ATM (fixed 53-byte cell)
          FDDI fiber dual-ring token passing · DQDB MAN (802.6)
 
   IEEE   802.3 Ethernet · 802.5 Token Ring · 802.6 DQDB · 802.11 Wireless LAN
 
   Addr   A 1~126 · B 128~191 · C 192~223 · D 224~239 (multicast)
          Private 10.x · 172.16~31.x · 192.168.x · Loopback 127.0.0.1
          Hosts = (256 − mask) − 2

Subnet Calculation One More Time

Mask	/Notation	Block	Hosts
255.255.255.0	/24	256	254
255.255.255.128	/25	128	126
255.255.255.192	/26	64	62
255.255.255.224	/27	32	30
255.255.255.240	/28	16	14
255.255.255.248	/29	8	6
255.255.255.252	/30	4	2

   256 − last value of the mask = block size
   block size − 2 = number of usable hosts
   256 ÷ block size = number of subnets

Same IP, Different Mask

Keep the IP fixed at 192.168.1.130 and change only the mask. You can see at a glance that the longer the mask, the narrower the network the address belongs to.

Mask	Block	Network address	Broadcast address	Host range	Count
/25	128	.128	.255	.129 ~ .254	126
/26	64	.128	.191	.129 ~ .190	62
/27	32	.128	.159	.129 ~ .158	30
/28	16	.128	.143	.129 ~ .142	14
/29	8	.128	.135	.129 ~ .134	6
/30	4	.128	.131	.129 ~ .130	2

With every mask, 130 falls into the block that starts at 128, so the network address came out the same. What changes is the end of the block. The broadcast address is always network address + block size − 1.
/30 has exactly 2 hosts. It is the smallest subnet, used for a segment that links two devices one-to-one.

Are Two Addresses in the Same Subnet?

Two addresses	Mask	Boundary	Verdict
192.168.1.60 and 192.168.1.70	/25 (block 128)	Both within 0 ~ 127	Same subnet — direct communication
192.168.1.60 and 192.168.1.70	/26 (block 64)	60 is in 0 ~ 63, 70 is in 64 ~ 127	Different subnets — a router is needed
192.168.1.100 and 192.168.1.126	/27 (block 32)	Both within 96 ~ 127	Same subnet
192.168.1.126 and 192.168.1.127	/27 (block 32)	127 is the broadcast address of that block	127 cannot be assigned to a host

Numbers that are close can still be different networks if a boundary lies between them. 60 and 70 differ by only 10, but under /26 the boundary at 64 separates them.
When you are given an address, first check that it is not the first (network) or last (broadcast) address of its block. Questions that ask you to pick "an address that cannot be assigned to a host" take this form.

Threading Four Days Through One Connection

This is what happens when you type www.example.com into a browser on a computer whose IP is 192.168.0.10/24 and whose default gateway is 192.168.0.1.

   1  Name → IP          Check /etc/hosts; if it is not there, ask the DNS server (port 53) in /etc/resolv.conf
                         → receive 93.184.216.34                              (Day 4)
   2  Inside or outside?  My network is 192.168.0.0 ~ 192.168.0.255. 93.x is outside
                         → send it to the default gateway 192.168.0.1          (Days 3·4)
   3  IP → MAC           Find the gateway's MAC with ARP                       (Day 1)
   4  Encapsulation       Send it out as a [ MAC | IP | TCP | data ] frame      (Day 1)
   5  Path selection      Routers look at the IP and pass the packet to the next hop (Day 2, packet switching)
   6  Connect and request TCP 3-way handshake, then an HTTP request to port 80  (Day 1)

If this step breaks	Symptom you see	What to check
1	It works by IP but not by name	`/etc/resolv.conf`, `nslookup`
2	Hosts on the same network work but the outside does not	The `default` line of `ip route`
3	You cannot even reach the device right next to you	Cable, `arp -n`, IP and mask
6	`ping` works but only that one service does not	Whether the other side's port is open — `netstat -tulpn`

Three kinds of addresses are nested inside one frame. The port means "which program", the IP means "which computer", and the MAC means "which device is the very next hop".
That is why the destination IP stays the same all the way, while the MAC changes to the next segment's each time the packet passes a router. It is also why step 3 looks for the gateway's MAC, not the server's.
A switch looks only at the MAC in step 4, while a router opens up to the IP in step 5. A device's layer = the layer of the header it opens.

Putting Commands on the Layers

Command	What it shows	Layer · address it touches
`ethtool`	Link status, speed	Layer 1
`arp` · `ip neigh`	IP ↔ MAC table	Layer 2 — MAC
`ifconfig` · `ip addr`	Interface IP and MAC	Layers 2·3
`route` · `ip route`	Routing table, default gateway	Layer 3 — IP
`ping` · `traceroute`	Reachability, path	Layer 3 — ICMP
`netstat` · `ss`	Open ports, connections	Layer 4 — port
`nslookup` · `dig` · `host`	Name ↔ IP	Layer 7 — DNS

Sweeping from bottom to top gives you a troubleshooting order as is. Is the cable attached → is an address assigned → is there a route → is the port open → does the name resolve?

Comparing Easily Confused Pairs

Item	A	B	Distinguishing criterion
OSI vs TCP/IP	7 layers, theory	4 layers, practice	Purpose
Switch vs router	Layer 2, MAC	Layer 3, IP	Address it looks at
MAC vs IP	Hardware, fixed	Configured, changeable	Where it is engraved
TCP vs UDP	Reliable, connection-oriented	Fast, connectionless	What it prioritizes
Circuit vs packet switching	PSTN, dedicated	PSDN, split up	Path occupancy
Datagram vs virtual circuit	Connectionless	Connection-oriented, order kept	Whether the path is fixed
CSMA/CD vs CA	Wired, detection	Wireless, avoidance	Medium
X.25 vs Frame Relay	More error checking, slow	Trimmed down, fast	Assumes good line quality
Class A vs C	1~126	192~223	First octet
`/etc/hosts` vs `resolv.conf`	Name↔IP answers	DNS servers to ask	What gets written
`ifconfig` vs `ip`	Old net-tools	Current iproute2	Generation
`ping` vs `traceroute`	Can I reach it?	Where does it get blocked?	What it tells you

Commonly Missed Points

"Layer 5 is Transport" → Layer 4 is Transport; Layer 5 is Session.
"A switch is Layer 3" → It is Layer 2. Layer 3 is the router.
"UDP guarantees reliability" → It does not. TCP does.
"CSMA/CA is wired" → CA is wireless, and CD is wired.
"An ATM cell is 48 bytes" → It is 53 bytes. 48 is only the data portion.
"127 is Class A" → It is left out of the range. It is reserved for loopback.
"All of 172.x is private IP" → Only 172.16 ~ 172.31 is.
"255.255.255.192 means 64 hosts" → It is 62. You must subtract 2.
"A mask like 255.255.255.100" → It cannot exist. The 1 bits must be contiguous from the left.
"Write the DNS server in /etc/hosts" → It goes in /etc/resolv.conf.
"IPv6 is 64 bits" → It is 128 bits. IPv4 is 32 bits.

Telling Look-Alike Names Apart

Look-alikes	One line to tell them apart
ISO / OSI	ISO is the organization, OSI is the 7-layer model that organization made
IEEE / IETF	IEEE sets the 802 (LAN) standards, IETF produces RFCs (Internet protocols)
IANA / ICANN	Both manage addresses and names. IANA does the hands-on number assignment, ICANN oversees
PSTN / PSDN	T is telephone (circuit switching), D is data (packet switching)
ARP / RARP	IP → MAC / MAC → IP
FDDI / DQDB	Dual ring (fiber) / dual bus (MAN)
Token Ring / FDDI	Both use token passing. 802.5 / fiber dual ring
Hub / Switch	Floods to everyone (Layer 1) / looks at the MAC and sends only to that port (Layer 2)
Network address / Broadcast address	All host bits 0 / all host bits 1
`netstat -r` / `netstat -i`	Routing table / interface statistics
`ip addr` / `ip link`	Handles addresses / turns devices on and off

Telling Look-Alike Numbers Apart

Number	Confusing pair	How to tell them apart
48	48 data bytes of an ATM cell / 48-bit MAC address	Different units — bytes vs bits
53	53-byte ATM cell / DNS port 53	The same number by coincidence
64 / 62	Address count / host count of `/26`	Whether you subtracted 2
126	Last first octet of Class A / host count of `/25`	Range vs count
127	Loopback range	The value missing between A and B
802.3 / 802.5 / 802.11	Ethernet / Token Ring / wireless LAN	3 is wired CSMA/CD, 11 is wireless CSMA/CA

Values That Appear on the Exam as They Are

Value	What it is
7 / 4	Number of OSI layers / number of TCP/IP layers
53 bytes	ATM cell size
62	Host count of `/26` (255.255.255.192)
32 / 128 bits	IPv4 / IPv6 address length
48 bits	MAC address length
127.0.0.1	Loopback
0~1023	Well-known port range
22 / 23 / 25 / 53 / 80 / 110	SSH / TELNET / SMTP / DNS / HTTP / POP3

Next week is Internet services. It covers the web, mail, DNS, remote access, and file sharing. 20 of the 32 Linux Usage questions come from this area, so it carries the greatest weight.

📖 Glossary

OSI 7 layers : A reference model that divides communication into seven stages, from Physical to Application.
CSMA/CD : The wired Ethernet method that detects collisions and retransmits.
ATM : A transmission method that uses fixed 53-byte cells.
Subnet mask : A value that separates the network portion from the host portion.
Private IP : Addresses used only inside an internal network. The 10, 172.16~31, and 192.168 ranges.
Default gateway : The router that traffic leaving the network passes through.

📝 Practice Questions

Click a choice to reveal the answer and explanation.

Question 1

In the OSI 7 layers, which layer does a switch operate at?

Question 2

Which medium access control method is used in wireless LANs?

Question 3

Which of the following is the loopback address that points to the host itself?

Question 4

When the subnet mask 255.255.255.224 is applied to a Class C network, how many usable hosts does one subnet have?

Question 5

What is the default port number of SMTP, which is used to send mail?

Question 6

Which of the following is not in a private IP address range?

Official references

Linux Master 2 official exam syllabus(opens in a new tab)