Subnet Mask - Explained

Subnet masks: what they are and why they matter

IP address basics (IPv4)

• An IPv4 address is 32 bits, written as four octets (each ranges from 0–255).
• An IPv4 address is split into two parts:
  • Network portion (network address / network ID)
  • Host portion (host address / host ID)

A subnet mask is a value (similar-looking to an IP) that determines which bits of the IP belong to the network vs. the host.

How subnet masks work (binary + alignment)

Computers work in binary (1s and 0s). The explanation uses an 8-bit octet chart (bit values: 1, 2, 4, 8, …, 128) to convert dotted decimal subnet masks into binary.

Rule for identifying network bits:

• In the subnet mask, 1s indicate positions that belong to the network
• 0s indicate positions that belong to the host

Examples (conceptual):

• If the subnet mask is 255.255.255.0:
  • first 3 octets = network
  • last octet = host
• If the subnet mask is 255.255.224.0:
  • the network portion spans the first two octets plus the first 3 bits of the third octet
  • the remaining bits are host
• If the subnet mask is 255.0.0.0:
  • the first octet = network
  • the remaining octets are host

Why network vs host exists (broadcast/control)

IP addresses include a network part so networks can be managed via subnetting (splitting a large network into smaller ones).

Without subnetting, a large network leads to excessive broadcast traffic:

• Devices send broadcasts to discover or communicate
• Broadcasts reach all devices on that network, which can cause congestion and make problems harder to isolate

Routers prevent broadcast propagation:

• Broadcasts don’t pass through routers
• Devices send traffic to a default gateway/router when the destination is on another subnet

Subnetting tutorial example (small business)

Scenario: A small business has 12 computers, initially all in one network, and wants 3 separate departmental networks so departments don’t see each other’s traffic.

Key subnetting method described:

• Borrow bits from the host portion to create more subnetworks.
• To create a custom mask, you change the default subnet mask by turning some host-side 0s into 1s.

Bit-borrowing outcomes (as described):

• Borrow 1 bit → 2 subnets (fewer hosts per subnet)
• Borrow 2 bits → 4 subnets (even fewer hosts per subnet)
• Borrow up to 6 bits to illustrate the tradeoff:
  • more subnets as more host bits are borrowed
  • fewer hosts per subnet as more bits are borrowed

For 3 networks:

• The lesson indicates you’d need to borrow 2 bits, which produces enough subnets (noted as 4 in the simplified model).
• The final recommended custom mask stated is: 255.255.255.192 (sufficient to support 3 networks).

IP class overview (A–E) and default masks

The explanation mentions that IPv4 addresses have 5 classes (A–E), but focuses on A, B, and C due to common commercial defaults.

Class selection is determined by the first octet and its default subnet mask:

• Class A: very large host capacity (example given: up to ~16 million hosts)
• Class B: medium-large (~65,000 hosts)
• Class C: smaller (~254 hosts; common for small organizations/homes)

CIDR / slash notation (alternative subnet mask format)

Subnet masks can also be written using CIDR / slash notation:

• The /number equals the count of 1s in the subnet mask.

Examples mentioned:

• /24 → 24 bits of subnet mask
• /25, /26, /8 → imply different subnet mask lengths

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Main speakers / sources

• The explanation is presented by a single primary speaker/creator, with no external sources or named experts referenced.
• Mentions an Amazon Audible audiobook promotion (an audio book is recommended), but no author name is provided in the subtitles.