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VPC Basics Networking Fundamentals

IP Addressing

  • An IP (Internet Protocol) address is the unique identity of each host in a network.

  • Two versions of IP addresses are used:

    • IPv4 – 32-bit address (4 octets × 8 bits)
    • IPv6 – 128-bit address

Each IPv4 octet ranges from 0–255.

Example

IP Address: 192.168.56.212

Octet Decimal Binary Breakdown
1 192 128 + 64 = 2⁷ + 2⁶
2 168 128 + 32 + 8 = 2⁷ + 2⁵ + 2³
3 56 32 + 16 + 8 = 2⁵ + 2⁴ + 2³
4 212 128 + 64 + 16 + 4 = 2⁷ + 2⁶ + 2⁴ + 2²

CIDR (Classless Inter-Domain Routing)

CIDR is a method of allocating IP addresses and defining network ranges.

A CIDR block is represented as:

IP Address / Prefix Length

Examples:

10.0.0.0/16
10.0.100.0/24
192.168.0.0/32

AWS Note: When creating an IPv4 VPC, the primary IPv4 CIDR block must have a prefix length between /16 (largest network) and /28 (smallest network). In contrast, CIDR notation itself supports prefix lengths from /0 to /32.

CIDR Components

Every CIDR block has two parts.

1. Base IP Address

Represents the network address (base address) of the CIDR block. It is the first IP address in the CIDR range and identifies the network itself.

Examples:

10.0.0.0
172.16.0.0
192.168.0.0

2. Prefix Length (Subnet Mask)

The prefix length determines:

  • Number of Network Bits
  • Number of Host Bits
Host Bits = 32 - Prefix Length

The host bits identify individual IP addresses within the network.

Understanding CIDR

Example 1: 10.0.0.0/16

Prefix Length = 16

Host Bits
= 32 - 16
= 16

Total IP Addresses
= 2^16
= 65,536

Since the first 16 bits are fixed:

10.0.x.x

the third and fourth octets can vary.

10.0.0.0
10.0.0.1
10.0.0.2
...
10.0.0.255

10.0.1.0
10.0.1.1
...
10.0.1.255

...

10.0.255.255

Example 2: 10.0.100.0/24

Prefix Length = 24

Host Bits
= 32 - 24
= 8

Total IP Addresses
= 2^8
= 256

Since the first 24 bits are fixed:

10.0.100.x

Only the last octet changes.

10.0.100.0
10.0.100.1
10.0.100.2
...
10.0.100.255

CIDR Reference

CIDR Host Bits Subnet Mask Total IPs Example IP Range
/32 0 255.255.255.255 1 (2⁰) 192.168.0.0
/31 1 255.255.255.254 2 (2¹) 192.168.0.0 – 192.168.0.1
/30 2 255.255.255.252 4 (2²) 192.168.0.0 – 192.168.0.3
/29 3 255.255.255.248 8 (2³) 192.168.0.0 – 192.168.0.7
/28 4 255.255.255.240 16 (2⁴) 192.168.0.0 – 192.168.0.15
/27 5 255.255.255.224 32 (2⁵) 192.168.0.0 – 192.168.0.31
/26 6 255.255.255.192 64 (2⁶) 192.168.0.0 – 192.168.0.63
/25 7 255.255.255.128 128 (2⁷) 192.168.0.0 – 192.168.0.127
/24 8 255.255.255.0 256 (2⁸) 192.168.0.0 – 192.168.0.255
/16 16 255.255.0.0 65,536 (2¹⁶) 192.168.0.0 – 192.168.255.255
/8 24 255.0.0.0 16,777,216 (2²⁴) 10.0.0.0 – 10.255.255.255
/0 32 0.0.0.0 4,294,967,296 (2³²) 0.0.0.0 – 255.255.255.255

Quick Memory Trick

/16

10.0.x.x

✓ First 2 octets remain fixed
✓ Last 2 octets change

/24

10.0.100.x

✓ First 3 octets remain fixed
✓ Last octet changes

/32

192.168.0.0

✓ No host bits
✓ Only one IP address

CIDR Formulas

Network Bits = Prefix Length

Host Bits = 32 - Prefix Length

Total IP Addresses = 2^(Host Bits)

Example:

192.168.0.0/24

Network Bits = 24
Host Bits = 8

Total Addresses = 2^8 = 256

IP Address Classes

Class Address Range
A 1.0.0.1 – 126.255.255.254
B 128.1.0.1 – 191.255.255.254
C 192.0.1.1 – 223.255.255.254
D 224.0.0.0 – 239.255.255.255

Note: Modern networks (including AWS VPCs) use CIDR, not classful addressing. IP classes are included only for historical understanding.

Private IP Ranges

Private IP addresses are used within private networks and cannot be routed over the Internet.

CIDR Address Range Typical Usage
10.0.0.0/8 10.0.0.0 – 10.255.255.255 Large Enterprise Networks
172.16.0.0/12 172.16.0.0 – 172.31.255.255 AWS VPCs commonly use this range
192.168.0.0/16 192.168.0.0 – 192.168.255.255 Home / Office Networks

All remaining IPv4 addresses are Public IP addresses.

Public vs Private vs Elastic IP

Feature Private IP Public IP Elastic IP
Typical Use Communication within the VPC/private network Internet communication Static Internet communication
Address Source Subnet CIDR AWS Public IPv4 Pool AWS Public IPv4 Pool
Retained After Stop/Start Yes No Yes
Released on Stop No Yes No
Released on Termination Yes Yes Only if manually released

AWS Reserved IP Addresses

AWS reserves the first four IP addresses and the last IP address in every subnet (5 IP addresses total).

Example subnet:

10.0.0.0/24
Reserved IP Purpose
10.0.0.0 Network Address
10.0.0.1 VPC Router
10.0.0.2 Amazon-provided DNS
10.0.0.3 Reserved for Future Use
10.0.0.255 Reserved by AWS (last IP address)

Note: Although the last IP address is traditionally used as the broadcast address in IPv4 networks, AWS VPC does not support broadcast traffic. AWS simply reserves this address.

Example:

10.0.0.0/24

Total IP Addresses  : 256
Reserved by AWS     : 5 IP addresses
Usable IP Addresses : 251

Traditional Networking vs AWS

Traditional Networking

Usable Hosts
= Total IP Addresses - 2

(Network Address + Broadcast Address)

Example

/24

256 - 2 = 254 usable hosts

AWS VPC

Usable IP Addresses
= Total IP Addresses - 5

(Network + Router + DNS + Future Use + Last Address)

Example

/24

256 - 5 = 251 usable IP addresses

VPC Scope

Scope AWS Services
Global Route 53, IAM, Billing
Regional VPC, Internet Gateway, NAT Gateway, Elastic Load Balancer (ELB)
Availability Zone Subnets, EC2 Instances, RDS Instances

Default AWS VPC Service Quotas

Resource Default Limit
VPCs per Region 5
Subnets per VPC 200
Internet Gateways 5
Elastic IPs 5
Route Tables 200
Routes per Route Table 50
Network ACLs 200
Rules per NACL 20 Inbound + 20 Outbound
Security Groups 2,500
Rules per Security Group 60 Inbound + 60 Outbound
Security Groups per ENI 5
ENIs per Region 350
DHCP Option Sets 1
VPN Connections 50
Customer Gateways 50
VPC Peering Connections 50
Flow Logs 2 per ENI / Subnet / VPC

Note: These are common default AWS service quotas. Actual quotas may vary by AWS account, Region, and service updates. Many quotas can be increased by requesting a quota increase.

Important Formulas

IPv4 Size = 32 Bits

Network Bits = Prefix Length

Host Bits = 32 - Prefix Length

Total IP Addresses = 2^(Host Bits)

Traditional Usable Hosts = Total IP Addresses - 2

AWS Usable IP Addresses = Total IP Addresses - 5