📘 Networking Summary Guide

I. What is Networking?

Computer Networking refers to the interconnection of computers and devices that share data and resources. It underpins nearly all IT and DevOps systems, including cloud infrastructure, server communication, and service discovery.

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II. Basic Networking Concepts

Term	Description
IP Address	Unique identifier for a device on a network. IPv4 (e.g., 192.168.1.1), IPv6 (e.g., fe80::1).
MAC Address	Hardware address of a NIC (e.g., 00:1A:2B:3C:4D:5E).
Subnet	Logical segmentation of a network (e.g., /24 = 255.255.255.0).
Gateway	A router that connects a subnet to external networks.
DNS	Domain Name System – resolves domain names to IP addresses.
DHCP	Dynamic Host Configuration Protocol – assigns IPs automatically.

III. OSI Model (7 Layers)

Layer	Name	Function
7	Application	User interaction (HTTP, FTP, DNS)
6	Presentation	Data formatting (SSL, JPEG)
5	Session	Session management
4	Transport	Data delivery (TCP/UDP)
3	Network	Routing (IP)
2	Data Link	MAC addressing, frames
1	Physical	Hardware, cables, NICs

IV. Key Protocols

Protocol	Layer	Purpose
IP	3	Addressing and routing
TCP	4	Reliable communication
UDP	4	Faster, connectionless communication
HTTP/HTTPS	7	Web communication
DNS	7	Name resolution
SSH	7	Secure shell access
ICMP	3	Ping/traceroute functionality
FTP/SFTP	7	File transfer

V. Networking Tools (Linux CLI)

Command	Usage
ip a	Show network interfaces & IPs
ping	Test connectivity
traceroute	Trace packet path
netstat -tuln or ss -tuln	Show listening ports
dig / nslookup	DNS resolution info
curl / wget	HTTP requests
tcpdump	Network packet capture
nmap	Network scanning

VI. Private vs Public IP

• Private IP ranges (RFC1918):

  • 10.0.0.0/8

  • 172.16.0.0/12

  • 192.168.0.0/16

• Public IP: Routable on the internet.

• NAT (Network Address Translation): Maps private to public IPs.

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VII. TCP vs UDP

Feature	TCP	UDP
Connection	Connection-oriented	Connectionless
Reliability	Guaranteed, ordered delivery	No guarantee/order
Speed	Slower	Faster
Use Cases	Web, email, SSH	DNS, video streaming

VIII. Port Numbers

Port	Service
22	SSH
53	DNS
80	HTTP
443	HTTPS
25	SMTP
3306	MySQL
5432	PostgreSQL
6379	Redis
8080	HTTP-alt (apps)

IX. CIDR Notation & Subnetting

CIDR	Subnet Mask	Hosts
/8	255.0.0.0	~16M
/16	255.255.0.0	~65K
/24	255.255.255.0	254
/32	255.255.255.255	1

X. Firewalls and Security

• iptables or nftables: Linux-based firewall tools.

• UFW (Uncomplicated Firewall): Easy firewall management.

• Security best practices:

  • Allow only required ports.

  • Block unused protocols.

  • Use VPNs for private access.

  • Monitor traffic with IDS/IPS (Snort, Suricata).

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XI. DNS Concepts

Term	Description
A Record	Maps domain to IPv4 address
AAAA	Maps domain to IPv6
CNAME	Alias to another domain
MX	Mail exchange server
NS	Nameservers for a domain
TTL	Time to live (cache duration)

XII. Network Services in DevOps

• Load Balancers: Distribute traffic across servers (e.g., NGINX, HAProxy).

• Reverse Proxies: Handle incoming requests and forward to services.

• Service Discovery: Identify service locations dynamically (e.g., Consul, Kubernetes DNS).

• Overlay Networks: Used in containers for cross-node communication (Flannel, Calico).

• VPNs: Secure access to private networks (WireGuard, OpenVPN).

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XIII. Common Network Troubleshooting Steps

1. Check IP: ip a

2. Check DNS: dig google.com

3. Test ping: ping 8.8.8.8

4. Route check: traceroute google.com

5. Port check: ss -tuln, nmap localhost

6. Logs: /var/log/syslog, /var/log/messages

7. Capture traffic: tcpdump -i eth0

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XIV. Best Practices

1. Use private networks and subnets for internal services.

2. Apply strict firewall rules.

3. Monitor traffic with tools like Netdata, Nagios, Prometheus.

4. Encrypt data in transit (TLS, VPN).

5. Use role-based access for network devices.