In Linux networking, port numbers are 16-bit unsigned integers, which means the valid range is from 0 to 65535. This gives us:

Minimum port: 0
Maximum port: 65535

The entire port range is divided into three categories:

• Well-known ports (0-1023): Reserved for system services (HTTP, SSH, FTP, etc.)
• Registered ports (1024-49151): Assigned to specific applications by IANA
• Dynamic/private ports (49152-65535): Available for temporary use

When configuring services to use non-standard ports, consider these technical aspects:

# Example of setting SSH to port 22222 in /etc/ssh/sshd_config
Port 22222

Best practices for port selection:

• Avoid ports below 1024 (requires root privileges)
• Choose memorable numbers within the upper range
• Check for conflicts with netstat -tuln or ss -tuln

While 65535 is the theoretical maximum, some practical limitations exist:

# Kernel parameter affecting port range
sysctl net.ipv4.ip_local_port_range
# Typical output: 32768 60999

This shows the ephemeral port range used for outgoing connections.

Here's how to validate a port number in shell scripts:

#!/bin/bash

validate_port() {
    local port=$1
    if [[ ! $port =~ ^[0-9]+$ ]] || [ $port -lt 0 ] || [ $port -gt 65535 ]; then
        echo "Error: $port is not a valid port number (0-65535)"
        return 1
    fi
    return 0
}

# Usage example:
validate_port 8080 || exit 1

When using non-standard ports, remember to update firewall rules:

# Example for UFW (Uncomplicated Firewall)
sudo ufw allow 22222/tcp
sudo ufw allow 50000:60000/udp

In Linux systems, TCP/UDP port numbers are 16-bit unsigned integers, which means the theoretical maximum port number is 65,535 (2^16 - 1). However, there are important practical considerations:

# To verify the maximum allowed port number programmatically:
cat /proc/sys/net/ipv4/ip_local_port_range
# Typical output: 32768   60999

The port space is divided into three ranges:

• Well-known ports: 0-1023 (requires root privileges)
• Registered ports: 1024-49151
• Dynamic/private ports: 49152-65535

When choosing non-standard ports for services, consider these constraints:

# Example of setting a high port for SSH in /etc/ssh/sshd_config
Port 54321
# Then restart the service:
sudo systemctl restart sshd

For production systems, follow these guidelines:

• Avoid ports below 1024 unless necessary
• Check IANA registered ports list for conflicts
• Document all custom port assignments
• Update firewall rules accordingly

# Verify port availability with netstat:
netstat -tulnp | grep ':54321'
# Or using ss:
ss -tulnp | grep '54321'

To modify the ephemeral port range (affects client connections):

# Temporary change:
echo "50000 60000" > /proc/sys/net/ipv4/ip_local_port_range
# Permanent change in /etc/sysctl.conf:
net.ipv4.ip_local_port_range = 50000 60000

Common issues and solutions:

• Check SELinux contexts when services fail to bind
• Verify firewall rules with iptables/nftables
• Test connectivity with telnet/nc

# Example connectivity test:
nc -zv localhost 54321
# Or using telnet:
telnet localhost 54321