Network - Mirktech

Jun 22, 2025

0 Networking

How to Reset Cisco 1921/1941 Routers

After you connect your router with the console cable, you will need to use your SSH program like PuTTY or, in my case, Tera Term. You will need a rollover cable to connect your PC and router/switch.

Turn on the router and make a connection with the serial port in Tera Term. In order to enter the ROMmon CLI, go to Control Send break (Alt + B).

Next, enter "confreg 0x2142", then enter "reset".
You can skip setup by entering "no".

Then type "en" and "erase startup-config".
You will need to revert the config-register by typing "config t", next "config-register 0x2102", then "end", and finally type "reload".

May 10, 2025

0 Networking

Subnetting made easy – How to subnet a network

What is a subnet?

When we talk about subnet, we need to know about IPv4 address (Internet Protocol version 4).

An IP address is a 32-bit address, typically written as four dotted decimal octets (each octet is 8 bits). For example:
192.168.1.1

A network is defined by a subnet mask, which is also a 32-bit value. It can be written in dotted decimal format like 255.255.255.0 or in CIDR* (slash) notation like /24. The /24 means that the first 24 bits are used for the network portion, and the remaining 8 bits are used for host addresses within that network.

Let’s take an example:

IP address: 192.168.1.1
Subnet mask: /24 or 255.255.255.0

This means:

The network address is 192.168.1.0
(because the first 24 bits identify the network)
The range of usable host addresses is from 192.168.1.1 to 192.168.1.254
There are 256 total IPs in the subnet (2^8), but only 254 usable IP addresses because:
- 192.168.1.0 is the network address
- 192.168.1.255 is the broadcast address

*CIDR makes routing and IP management more flexible than the old class-based system (Class A, B, C). Instead of being forced into fixed subnet sizes, you can allocate IP blocks that exactly match your needs.

Apr 26, 2025

0 Networking Security

The most common ports used in networking

Understand essential TCP and UDP ports for CCNA,CompTIA A+/Network+, security, and real-world troubleshooting.

There are two transport layer protocols that we use in Layer 4 (Transport) OSI model:

TCP – Connection-oriented protocol
UDP – Connectionless protocol

TCP 20 – FTP (File Transport Protocol) Data transfer

TCP 21 – FTP (File Transport Protocol) Control command

TCP 22 – SSH (Secure Shell)

TCP 23 – Telnet

TCP 25 – SMTP (Simple Mail Transfer Protocol)

UDP 53 – DNS (Domain Name System)

UDP 67/68 – DHCP server/client ports

UDP 69 – TFTP (Trivial File Transfer Protocol)

TCP 80 – HTTP (Hypertext Transfer Protocol)

TCP 110 – POP3 (Post Office Protocol 3)

UDP 123 – NTP (Network Time Protocol)

TCP 143 – IMAP4 (Internet Message Access Protocol 4)

TCP 389 – LDAP (Lightweight Directory Access Protocol)

TCP 443 – HTTPS (Hypertext Transfer Protocol over Secure Socket Layer)

UDP 514 – Syslog (Logging Service)

TCP 993 – IMAPS (Internet Message Access Protocol over TLS/SSL)

TCP 995 – POP3S (Post Office Protocol 3 over TLS/SSL)

TCP 3389 – RDP (Remote Desktop Protocol)

Knowing which ports correspond to which services helps you troubleshoot network issues faster, secure your systems better, and pass certification exams with confidence.
Keep practicing and memorizing these ports—they are a fundamental part of real-world networking!