Inventory: From CSV to Ansible Inventory

The same fleet, one more time

Every capstone since "Your First Network Automation" has used the same five-device fleet:

devices.csv — unchanged since Phase 2.6

hostname,ip,platform
SW1,10.10.10.1,cisco_ios
SW2,10.10.10.2,cisco_ios
SW3,10.10.10.3,cisco_ios
SW4,10.10.10.4,cisco_ios
SW5,10.10.10.5,cisco_ios

Three columns: hostname, ip, platform. This lesson translates exactly this data into the two inventory formats Ansible understands — nothing about the underlying fleet changes.

Ansible doesn't read CSV directly. It reads its own inventory format — in either of two shapes: INI-style, or YAML.

Quick checkConcept check

Does Ansible read a devices.csv file directly as its inventory?

Yes, CSV is Ansible's native inventory formatNo — Ansible uses its own INI-style or YAML inventory format; devices.csv's data has to be translated into one of thoseYes, but only if the CSV has exactly three columnsNo, Ansible only supports YAML, never INI

INI-style inventory

inventory.ini

[switches]
SW1 ansible_host=10.10.10.1
SW2 ansible_host=10.10.10.2
SW3 ansible_host=10.10.10.3
SW4 ansible_host=10.10.10.4
SW5 ansible_host=10.10.10.5

[switches:vars]
ansible_network_os=ios
ansible_connection=network_cli

[switches] declares a group containing all five hosts. ansible_host maps each hostname to its IP — exactly devices.csv's hostname and ip columns. [switches:vars] applies shared variables to every host in the group at once, instead of repeating them per host.

platform becomes ansible_network_os — and the value itself changes too

devices.csv's platform column held Netmiko's device-type string: "cisco_ios". Ansible's own convention for the same fact is the variable ansible_network_os, and its value for Cisco IOS devices is "ios" — shorter, and specific to Ansible's own network modules. Same fact (this is a Cisco IOS device), two different tools' own naming conventions for it.

Quick checkIdentify the correct code

Which line correctly assigns SW3's management IP in INI-style inventory?

SW3 = 10.10.10.3SW3 ansible_host=10.10.10.3ansible_host: SW3=10.10.10.3[SW3] ansible_host=10.10.10.3

YAML inventory — the same data, Ansible's other format

inventory.yml

switches:
hosts:
  SW1:
    ansible_host: 10.10.10.1
  SW2:
    ansible_host: 10.10.10.2
  SW3:
    ansible_host: 10.10.10.3
  SW4:
    ansible_host: 10.10.10.4
  SW5:
    ansible_host: 10.10.10.5
vars:
  ansible_network_os: ios
  ansible_connection: network_cli

Identical information to inventory.ini above — a switches group, five hosts each with an ansible_host, and shared vars for the whole group. This is real, valid YAML — worth reading closely against 'Understanding YAML for Network Automation''s indentation rules from Phase 2.5.

Reading this, not writing a parser for it

This is the same YAML-reading skill Phase 2.5 built — indentation shows nesting, a colon-and-space separates key from value. What's different here is what reads it: Ansible itself parses this file when a playbook runs. Nothing in this module writes Python code to parse it — that was a deliberate choice explained further in Lesson 4.

Quick checkPredict output

In the YAML inventory above, which hosts belong to the switches group?

Only SW1, since it's listed firstSW1 through SW5 — all five hosts nested under switches.hostsNone — hosts must be listed at the top level, not nested under a groupOnly hosts that also appear under vars

Groups and variables, briefly

A group (switches, here) is a named collection of hosts a playbook can target all at once, instead of listing five hostnames every time. A host variable (ansible_host) belongs to one specific host. A group variable (ansible_network_os, ansible_connection, under vars) applies to every host in the group — set once, not five times.

Quick checkScenario

A sixth switch, SW6, needs to be added to this fleet with a different connection type than the rest. Where does that difference belong?

As a group variable, since group variables apply to every hostAs a host variable on SW6 specifically, since it overrides what the group variable would otherwise applyIt can't be done — every host in a group must share identical variablesBy creating a new devices.csv file

Key takeaways

Ansible reads its own inventory format — INI-style or YAML — not CSV directly; devices.csv's exact data (hostname, ip, platform) translates into either.
platform's value changes from Netmiko's 'cisco_ios' to Ansible's own 'ios' convention for ansible_network_os — same fact, different tool's naming.
Groups let a playbook target many hosts at once; group variables apply to every host in a group; host variables override the group default for one specific host.
This is the same 5-device fleet (SW1–SW5) from every prior capstone — only the file format describing it has changed.

What you'll learn

The same fleet, one more time

INI-style inventory

YAML inventory — the same data, Ansible's other format

Groups and variables, briefly