Network Configuration and Verification

Why Static IP Configuration Matters in a Data Center

In a production data center, every server must have a predictable, permanent network address. Dynamic addressing (DHCP) is convenient for workstations, but servers need static IP addresses so that other systems—monitoring tools, backup agents, load balancers, and administrators—can always reach them at the same address. Configuring and verifying that address correctly is one of the first hands-on tasks a data center technician performs when racking a new server.

Core Network Parameters

Every Linux server connected to a network needs four pieces of information:

• IP address – the unique Layer 3 address assigned to the interface (for example, 10.10.20.50)
• Subnet mask – defines which portion of the address identifies the network versus the host (for example, 255.255.255.0, written as /24 in CIDR notation)
• Default gateway – the IP address of the router the server uses to reach other networks
• DNS resolver – the IP address of the name server that translates hostnames into IP addresses

Without a correct gateway the server cannot reach anything outside its own subnet. Without a working DNS entry it cannot resolve hostnames, even if IP connectivity is fine.

Viewing the Current Configuration

Before changing anything, always check what is already configured. The `ip` command is the standard tool on modern Linux distributions.

• `ip addr show` – lists every network interface and its assigned addresses
• `ip route show` – displays the routing table, including the default gateway (look for the line beginning with `default via`)

Read the output carefully. The interface name (for example, `ens3`, `eth0`, or `enp3s0`) varies by system and driver.

Configuring a Static IP on RHEL/AlmaLinux with nmcli

Red Hat–family distributions use NetworkManager, controlled from the command line with `nmcli`. The connection name is typically the interface name or a profile name shown by `nmcli connection show`.

A typical sequence to set a static address on a connection named `ens3`:

```

nmcli connection modify ens3 ipv4.method manual \

ipv4.addresses 10.10.20.50/24 \

ipv4.gateway 10.10.20.1 \

ipv4.dns 10.10.20.5

nmcli connection up ens3

```

After running these commands, use `ip addr show ens3` and `ip route show` to confirm the settings took effect.

Configuring a Static IP on Ubuntu Server with Netplan

Ubuntu Server uses Netplan, which reads YAML configuration files stored in `/etc/netplan/`. Edit (or create) the file for your interface, being careful with indentation—YAML is whitespace-sensitive:

```yaml

network:

version: 2

ethernets:

ens3:

dhcp4: no

addresses:

• 10.10.20.50/24

routes:

• to: default

via: 10.10.20.1

nameservers:

addresses:

• 10.10.20.5

```

Apply the configuration with `sudo netplan apply`. Then verify with `ip addr show` and `ip route show`.

Verifying Connectivity Step by Step

Always verify in order from the most local to the most remote:

1. Ping the gateway (`ping 10.10.20.1`) – confirms Layer 3 connectivity on the local subnet

2. Ping a remote IP – confirms routing is working

3. Ping a hostname (`ping server2.lab.local`) – confirms DNS is resolving

4. Use `traceroute` – shows each hop to a destination; useful when a ping fails and you need to find where the path breaks

5. Use `nslookup` or `dig` – queries DNS directly; isolates name-resolution failures from general connectivity failures

6. Use `ss -tuln` – lists open TCP and UDP ports; confirms which services are listening and on which addresses

Common Faults and How to Isolate Them

| Symptom | Likely Cause | First Check |

|---|---|---|

| Can ping gateway, cannot reach other subnets | Wrong gateway IP | `ip route show` |

| Can ping by IP, cannot ping by name | DNS misconfigured or unreachable | `nslookup hostname` or `dig hostname` |

| Ping blocked from outside, services running | Firewall blocking ICMP | Check firewall rules; use `ss` to confirm service is listening |

Documentation as a Professional Practice

After configuring a server, document every setting in your lab log and, in a real facility, in the Data Center Infrastructure Management (DCIM) system. A complete record includes: hostname, interface name, IP address and mask, gateway, DNS servers, and open ports (from `ss -tuln`). Accurate records prevent IP conflicts, speed up troubleshooting, and are required by most data center operating procedures.

Safety Note

This lab uses live network switches on a dedicated VLAN. Do not change VLAN assignments or switch port settings without instructor approval—misconfiguration can disrupt other students' connectivity. When working inside server chassis to seat a network card, follow lockout/tagout procedures and wear appropriate PPE (ESD wrist strap, safety glasses). For cable work in overhead trays, use a ladder rated for the load and follow your facility's ladder safety policy.