OIDs in SNMP: A Human-Friendly Guide to SNMP Numbers

 

First, what is an OID?

An OID (Object Identifier) is a unique numeric address used by SNMP to identify one specific piece of information on a device.

An OID is basically a house address for data inside a device.

Not the city.
Not the street.
The exact house, exact door, exact room where one tiny piece of information lives.

If SNMP is the language,
OIDs are the addresses where answers are stored.

Why OIDs exist (because computers are picky)

Humans say: “CPU usage”

Computers say: “Which CPU usage? User? System? Idle? Core 0? Core 47?”

So SNMP says: “Fine. Everything gets a number. No confusion. No feelings.”

Thus, OIDs were born.

How OIDs are structured (the family tree from hell)

An OID looks like this:

1.3.6.1.4.1.2021.11.9.0

At first glance:

“Is this a phone number? A password? A cry for help?”

Actually, it’s a tree path.

Each number is just:

“Go one step deeper.”

Let’s walk an OID like a human

1        → The root (the universe begins here)

1.3      → Organizations

1.3.6    → US Department of Defense (yes, really)

1.3.6.1  → The Internet

Congratulations 🎉
You are now officially inside the internet.

The two big SNMP neighborhoods

1️⃣   Standard stuff (everyone agrees)

1.3.6.1.2.1 → mib-2

This is where basic, boring, but useful information lives:

• System name
• Uptime
• Network interfaces
• Errors
• Counters

Example:

1.3.6.1.2.1.1.3.0 → System uptime

Translation:

“How long have you been alive without rebooting?”

2️⃣   Vendor-specific chaos

1.3.6.1.4.1 → enterprises

This is where vendors say:

“I’ll do my own thing.”

After this, each vendor gets a number:

• Linux (Net-SNMP): 2021
• Cisco: 9
• HP: 11

So:

1.3.6.1.4.1.2021 → Linux internals

This is where:

• CPU stats
• Memory
• Disk
• Load
• Weird vendor magic
  lives.

Scalar OIDs — “Just tell me ONE thing”

Scalar OIDs are single values.

They always end with:

.0

Why .0?
Because SNMP said:

“Trust me.”

Example:

1.3.6.1.2.1.1.5.0 → Hostname

Table OIDs — “Now tell me ALL of them”

Tables are for lists.

Example:

1.3.6.1.2.1.2.2 → Network interfaces

Each row has an index:

...1 → eth0

...2 → eth1

...3 → lo

So SNMP does:

“Same question, different row.”

Very organized.
Very boring.
Very powerful.

OIDs and MIBs — numbers vs sanity

Without MIBs:

1.3.6.1.4.1.2021.11.9.0

With MIBs:

ssCpuUser.0

Same thing.

MIBs are basically glasses for your eyes so you don’t go blind reading numbers all day.

Why OIDs feel evil at first

• They are long
• They are numeric
• They don’t forgive typos
• One wrong digit = wrong universe

But after a while:

“Oh, that’s under enterprises → Linux → CPU.”

Your brain adapts.
Your soul slightly withers.

How SNMP tools actually use OIDs

snmpget (polite question)

snmpget server sysUpTime.0

“Hey, quick question.”

snmpwalk (interrogation)

snmpwalk server 1.3.6.1.4.1.2021

“Tell me everything you know.”

Finding OIDs (the treasure hunt)

Ways humans find OIDs:

• Read vendor MIB files (brave souls)
• Use snmpwalk
• Google + copy-paste
• Ask senior admins
• Trial and error + regret

Important survival tips

• Scalars end in .0
• Tables use indexes
• 1.3.6.1.2.1 = standard
• 1.3.6.1.4.1 = vendor madness
• Printers have the weirdest OIDs

One-line human explanation

An OID is the GPS coordinate of a single fact inside a device.

Final admin wisdom

If SNMP is:

“Talking to machines”

Then OIDs are:

“Knowing exactly where to point and ask.”