Quick review: Networking – CCNA

What you really need to know

The Networking topic is the general review of fundamental CCNA concepts. Here you are not studying a single isolated topic, but how all parts of the network work together: OSI and TCP/IP models, Ethernet, switching, IP, subnetting, routing, IP services, wireless and basic security.

For CCNA, you need to read a network as a complete system. You need to understand at which layer a problem occurs, which device is involved, which protocol is relevant and which command or reasoning process to use to verify the situation.

The central point is this: networking is not only memorizing acronyms, but understanding how data moves from one host to another through different layers, addresses, devices and protocols.

Key concepts

• OSI model: 7-layer model used to understand network functions.
• TCP/IP model: practical model used to describe Internet communication.
• Ethernet: Layer 2 technology used in wired LANs.
• MAC address: Layer 2 address used in Ethernet networks.
• IP address: Layer 3 address used to identify hosts and networks.
• Subnetting: division of an IP network into smaller subnets.
• Default gateway: router used by a host to reach external networks.
• VLAN: logical separation of a Layer 2 network.
• Routing: forwarding of packets between different networks.
• NAT/PAT: IP address translation, often for Internet access.
• ACL: rules that permit or deny traffic.
• Wireless: network communication through radio.
• IP services: services such as DHCP, DNS, NTP, SNMP, Syslog and SSH.

Differences not to confuse

Concept	Main meaning
MAC address	Layer 2 address
IP address	Layer 3 address
Switching	Forwarding inside LAN/VLAN
Routing	Forwarding between different networks
VLAN	Logical broadcast domain
Subnet	Logical IP division
DHCP	Assigns IP parameters
DNS	Resolves names to IP addresses
NAT	Translates addresses
ACL	Filters traffic
Wireless	Connection through radio

OSI model

The OSI model divides network communication into 7 layers.

Layer	Name	Main function
7	Application	Services used by applications
6	Presentation	Data format, encryption, encoding
5	Session	Session management
4	Transport	TCP, UDP, ports, reliability
3	Network	IP, routing
2	Data Link	Ethernet, MAC, switching
1	Physical	Cables, signals, connectors

For CCNA, the most important layers are often Layer 1, 2, 3 and 4.

Examples:

• disconnected cable = Layer 1;
• VLAN or MAC address = Layer 2;
• IP and routing = Layer 3;
• TCP/UDP ports = Layer 4.

TCP/IP model

The TCP/IP model is more practical and describes the real operation of IP networks.

It can be seen in 4 layers:

TCP/IP layer	Examples
Application	HTTP, DNS, DHCP, SSH
Transport	TCP, UDP
Internet	IP, ICMP
Network Access	Ethernet, Wi-Fi

For CCNA, you need to connect OSI and TCP/IP without confusing them. OSI is useful for reasoning and troubleshooting, while TCP/IP better describes the communication used in modern networks.

Encapsulation

Encapsulation is the process by which data is wrapped with additional information as it moves down the layers.

Simplified example:

• application data;
• TCP or UDP segment;
• IP packet;
• Ethernet frame;
• bits on the physical medium.

When traffic reaches the destination, the reverse process occurs, called decapsulation.

For CCNA, you need to understand that each layer adds or interprets specific information.

Ethernet

Ethernet is the most common technology in wired LANs.

It mainly operates at Layer 2 and uses Ethernet frames.

An Ethernet frame contains information such as:

• source MAC;
• destination MAC;
• protocol type;
• data;
• error check.

Ethernet switches use the MAC address table to decide where to forward frames.

MAC address

The MAC address is a Layer 2 address associated with a network card or interface.

Example format:

• 00:11:22:33:44:55.

In an Ethernet LAN, devices communicate locally using MAC addresses.

Important difference:

• MAC = local Layer 2 communication;
• IP = logical Layer 3 communication between networks.

When a host needs to communicate with another host in the same network, it uses ARP to learn the MAC associated with the destination IP.

ARP

ARP, Address Resolution Protocol, is used to find the MAC address associated with an IPv4 address in the same local network.

Example:

• a PC wants to send traffic to 192.168.1.20;
• it knows the IP but not the MAC;
• it sends an ARP request;
• the device with that IP replies with its MAC.

ARP is essential for IPv4 to work over Ethernet.

IPv4

IPv4 uses 32-bit addresses written in dotted decimal format.

Example:

• 192.168.1.10.

An IPv4 host needs at least:

• IP address;
• subnet mask;
• default gateway;
• DNS, if it needs to resolve names.

For CCNA, you need to distinguish host address, network, broadcast and gateway.

Subnet mask and prefix length

The subnet mask indicates which part of the IP address represents the network and which part represents hosts.

Example:

• 255.255.255.0 equals /24.

With /24:

• network: 192.168.1.0;
• valid hosts: 192.168.1.1 - 192.168.1.254;
• broadcast: 192.168.1.255.

For CCNA, subnetting is essential: you must calculate networks, available hosts, broadcast addresses and ranges.

Subnetting

Subnetting divides a network into smaller subnets.

It is used to:

• organize the network better;
• separate departments or services;
• reduce broadcast domains;
• use addresses more efficiently;
• apply different policies;
• simplify routing and troubleshooting.

Example: a company network can be divided into subnets for users, servers, voice, management and guests.

IPv6

IPv6 uses 128-bit addresses written in hexadecimal format.

Example:

• 2001:db8:abcd:1::10.

IPv6 was created to overcome IPv4 limitations and offers a much larger address space.

Important concepts:

• global unicast addresses;
• link-local;
• multicast;
• prefix length;
• neighbor discovery;
• absence of traditional IPv4 broadcast.

For CCNA, you need to know IPv6 basics and recognize main addresses and functions.

Default gateway

The default gateway is the router used by a host to reach networks outside its own subnet.

If a host wants to communicate with an IP in the same subnet, it sends directly to that device.

If the IP is in another subnet, it sends traffic to the default gateway.

Common problem: if the default gateway is wrong or missing, the host can communicate locally but not with external networks.

Switching

Switching concerns frame forwarding inside a LAN or VLAN.

A switch learns MAC addresses by observing incoming traffic and builds a MAC address table.

If it knows the port associated with the destination MAC, it forwards the frame only there. If it does not know it, it floods within the VLAN.

Switching is mainly Layer 2.

VLAN

A VLAN logically separates a Layer 2 network into multiple broadcast domains.

Example:

• VLAN 10 = users;
• VLAN 20 = servers;
• VLAN 30 = voice;
• VLAN 99 = management.

Devices in different VLANs do not communicate directly at Layer 2. To communicate between different VLANs, inter-VLAN routing is required.

Trunk

A trunk carries traffic for multiple VLANs between network devices.

The most common standard is 802.1Q, which adds a VLAN tag to the Ethernet frame.

Examples of trunk links:

• switch to switch;
• switch to router;
• switch to firewall;
• switch to access point;
• switch to hypervisor.

For CCNA, you need to distinguish access port and trunk port.

Routing

Routing allows communication between different networks.

A router or Layer 3 switch checks the routing table and chooses the best path based on the destination IP.

Important concepts:

• connected routes;
• static routes;
• default route;
• dynamic routing;
• OSPF;
• longest prefix match;
• administrative distance;
• metric.

If a route to the destination or a return route is missing, communication may fail.

TCP and UDP

TCP and UDP operate at Layer 4.

TCP is connection-oriented and provides reliability through mechanisms such as acknowledgments and retransmissions.

Common examples:

• HTTP/HTTPS;
• SSH;
• FTP;
• email.

UDP is simpler and does not establish a reliable connection like TCP.

Common examples:

• DNS;
• DHCP;
• VoIP;
• streaming;
• NTP.

For CCNA, you need to know that TCP is more reliable, while UDP is lighter and often used where speed or simplicity matters.

TCP/UDP ports

Ports identify applications and services at Layer 4.

Common examples:

Service	Port
HTTP	TCP 80
HTTPS	TCP 443
SSH	TCP 22
DNS	UDP/TCP 53
DHCP	UDP 67/68
SNMP	UDP 161
Syslog	UDP 514
NTP	UDP 123

ACLs can use ports to permit or block specific traffic.

DHCP

DHCP automatically assigns IP configurations to clients.

It can provide:

• IP address;
• subnet mask;
• default gateway;
• DNS;
• lease time.

If a client does not receive an IP address, the issue may involve the DHCP server, VLAN, trunk, relay, ip helper-address or connectivity.

DNS

DNS translates names into IP addresses.

If a host can reach an IP but not a name, the problem is probably DNS.

Example:

• ping 8.8.8.8 works;
• ping www.example.com does not work;
• possible DNS problem.

NAT and PAT

NAT translates IP addresses. PAT allows multiple internal hosts to share a single public IP using different ports.

They are often used to allow private networks to access the Internet.

Warning: NAT does not replace routing, ACLs or firewalls. Correct paths and policies are still required.

ACL

ACLs, Access Control Lists, permit or deny traffic based on criteria such as:

• source IP address;
• destination IP address;
• protocol;
• port;
• direction;
• interface.

ACLs are used for filtering, access control and basic security.

Common mistakes:

• wrong rule order;
• forgetting the final implicit deny;
• applying the ACL on the wrong interface;
• wrong direction, inbound instead of outbound or vice versa.

Wireless

Wireless networks use access points and radio communication.

Basic concepts:

• SSID;
• BSSID;
• 2.4 GHz;
• 5 GHz;
• channels;
• WPA2/WPA3;
• roaming;
• wireless controller;
• guest network.

In wireless problems, you need to think about both the radio side and the wired side: VLANs, trunks, DHCP, DNS, authentication and policies.

Basic security

In CCNA networking, you need to know basic security controls such as:

• strong passwords;
• SSH instead of Telnet;
• disabling unnecessary services;
• updating devices;
• using ACLs;
• segmenting with VLANs;
• protecting switch ports;
• DHCP snooping;
• Dynamic ARP Inspection;
• logging and monitoring.

Security is not a single command: it is a set of consistent controls.

Troubleshooting

Troubleshooting must be methodical.

Useful approach:

• identify the problem;
• understand what works and what does not;
• check Layer 1;
• check Layer 2;
• check Layer 3;
• check DNS, DHCP, ACLs and services;
• test;
• document the solution.

Useful commands:

• ping;
• traceroute;
• ipconfig or ifconfig;
• show ip interface brief;
• show interfaces;
• show vlan brief;
• show mac address-table;
• show ip route;
• show running-config;
• show access-lists;
• show cdp neighbors;
• show lldp neighbors.

Common quiz mistakes

• Confusing MAC address and IP address.
• Confusing switching and routing.
• Thinking that different VLANs communicate without routing.
• Forgetting the default gateway on hosts.
• Confusing DHCP and DNS.
• Thinking that NAT solves internal routing problems.
• Forgetting the implicit deny in ACLs.
• Applying ACLs in the wrong direction.
• Thinking that wireless is only radio signal and not also VLANs, DHCP and security.
• Skipping Layer 1 in troubleshooting.
• Forgetting that DNS problems can look like Internet problems.
• Confusing TCP and UDP.

Mini exam scenario

A PC has the correct IP address and subnet mask and can ping its default gateway. However, it cannot reach a server in another network.

Possible causes include:

• missing route;
• ACL blocking traffic;
• incorrect remote gateway;
• return traffic problem;
• firewall;
• incorrect intermediate routing.

This scenario shows why networking requires end-to-end reasoning, not only looking at the single host.

Mini checklist before the quiz

Before starting the quiz, you should be able to explain:

• the main layers of the OSI model;
• the difference between Layer 2 and Layer 3;
• what MAC address and IP address do;
• what ARP is used for;
• how a subnet mask works;
• what default gateway means;
• what a switch does;
• what a router does;
• why VLANs are needed;
• why inter-VLAN routing is needed;
• the difference between TCP and UDP;
• what DHCP and DNS do;
• what NAT and PAT are used for;
• how ACLs generally work;
• how to reason through network troubleshooting.