IP Addressing • Software Engineering Learning Paths

IPv4 Addresses#

A 32-bit number that uniquely identifies a device on a network, written as four decimal octets:

192.168.1.100
 │    │   │  │
 └────┴───┴──┴── Each octet: 0-255 (8 bits)

Binary: 11000000.10101000.00000001.01100100
Decimal:    192  .   168  .    1   .   100

Subnetting and CIDR#

CIDR Notation#

CIDR (Classless Inter-Domain Routing) notation: IP/prefix_length

The prefix identifies the network portion, the remaining bits identify the host:

10.0.1.50/24
├── Network: 10.0.1   (first 24 bits — fixed)
└── Host:    .50      (last 8 bits — variable)

Network address: 10.0.1.0    (all host bits = 0)
Broadcast:       10.0.1.255  (all host bits = 1)
Usable hosts:    10.0.1.1 – 10.0.1.254 (254 hosts)

Subnet Mask#

The subnet mask marks which bits are network (1) and which are host (0):

CIDR	Subnet Mask	Network Bits	Host Bits	Usable Hosts
/8	255.0.0.0	8	24	16,777,214
/16	255.255.0.0	16	16	65,534
/20	255.255.240.0	20	12	4,094
/24	255.255.255.0	24	8	254
/26	255.255.255.192	26	6	62
/28	255.255.255.240	28	4	14
/32	255.255.255.255	32	0	1 (single host)

Formula: Usable hosts = 2^(32 - prefix) - 2 (subtract network and broadcast addresses)

Subnetting Example#

Split 10.0.0.0/16 into four equal subnets:

Original: 10.0.0.0/16 (65,534 hosts)

Split into /18s (borrow 2 bits from host → 4 subnets):
  10.0.0.0/18    → 10.0.0.1   – 10.0.63.254   (16,382 hosts)
  10.0.64.0/18   → 10.0.64.1  – 10.0.127.254  (16,382 hosts)
  10.0.128.0/18  → 10.0.128.1 – 10.0.191.254  (16,382 hosts)
  10.0.192.0/18  → 10.0.192.1 – 10.0.255.254  (16,382 hosts)

Private vs Public IP Ranges#

Private Ranges (RFC 1918)#

Not routable on the internet — used inside organizations and cloud VPCs:

Range	CIDR	Size	Typical Use
10.0.0.0 – 10.255.255.255	10.0.0.0/8	16M addresses	Cloud VPCs, large orgs
172.16.0.0 – 172.31.255.255	172.16.0.0/12	1M addresses	Medium networks
192.168.0.0 – 192.168.255.255	192.168.0.0/16	65K addresses	Home/small office

Special Addresses#

Address	Purpose
127.0.0.0/8	Loopback (localhost)
169.254.0.0/16	Link-local (APIPA — no DHCP)
0.0.0.0	“Any” address (listen on all interfaces)
255.255.255.255	Limited broadcast

IPv6#

128-bit addresses — solves IPv4 exhaustion (4.3 billion → 340 undecillion):

Full:       2001:0db8:85a3:0000:0000:8a2e:0370:7334
Shortened:  2001:db8:85a3::8a2e:370:7334
            (leading zeros dropped, consecutive zero groups → ::)

IPv6 Address Types#

Type	Prefix	Purpose
Global unicast	`2000::/3`	Public internet (like public IPv4)
Link-local	`fe80::/10`	Same network segment only
Loopback	`::1`	Localhost
Multicast	`ff00::/8`	One-to-many

IPv4 vs IPv6#

Feature	IPv4	IPv6
Address size	32 bits	128 bits
Notation	Dotted decimal	Colon hexadecimal
NAT required	Yes (address scarcity)	No (enough addresses)
Header	Variable length	Fixed 40 bytes
ARP	Yes	Replaced by NDP

DHCP (Dynamic Host Configuration Protocol)#

Automatically assigns IP addresses to devices:

sequenceDiagram
    participant C as Client
    participant S as DHCP Server
    
    C->>S: DISCOVER (broadcast: "I need an IP")
    S->>C: OFFER (here's 192.168.1.50, lease 24h)
    C->>S: REQUEST (I'll take 192.168.1.50)
    S->>C: ACK (confirmed, it's yours for 24h)

DHCP provides: IP address, subnet mask, default gateway, DNS servers.

Cloud Networking Context#

In AWS VPCs:

VPC CIDR: 10.0.0.0/16

Subnets:
  Public:  10.0.1.0/24  (web servers, load balancers)
  Private: 10.0.10.0/24 (app servers, databases)
  
AWS reserves 5 IPs per subnet:
  .0 (network), .1 (gateway), .2 (DNS), .3 (reserved), .255 (broadcast)
  
/24 subnet: 256 - 5 = 251 usable IPs

Key Takeaways#

CIDR notation (/24, /16) defines network vs host boundary
Private ranges (10.x, 172.16-31.x, 192.168.x) are for internal use
Subnetting divides networks — essential for VPC design
AWS reserves 5 IPs per subnet — plan accordingly
IPv6 is the future but IPv4 + NAT dominates current infrastructure
Know your /24 = 254 hosts, /16 = 65K hosts for quick capacity planning