Quartz 4

IP Addressing

1 min read

https://jodies.de/ipcalc

🔐 IP Addressing & Subnetting Cheat Sheet

📦 Private IP Address Ranges (RFC 1918)

ClassRangeCIDR Notation (subnet mask)Size
A10.0.0.0 – 10.255.255.25510.0.0.0/8~16.7 million
B172.16.0.0 – 172.31.255.255172.16.0.0/12~1 million
C192.168.0.0 – 192.168.255.255192.168.0.0/16~65,000

All other IPs are considered public unless otherwise reserved.

🧠 IP Classes (Legacy Reference)

Class1st Octet RangeDefault Subnet MaskNotes
A1 – 126255.0.0.0 (/8)Very large networks
B128 – 191255.255.0.0 (/16)Medium networks
C192 – 223255.255.255.0 (/24)Small networks
D224 – 239N/AMulticast only
E240 – 255N/AExperimental / Reserved

127.0.0.1 is not Class A; it’s loopback only.

⚙️ CIDR to Host Counts

CIDRTotal IPsUsable HostsBlock SizeNotes
/30424Point-to-point links
/29868Small device subnets
/28161416Common in VLANs
/27323032
/26646264
/25128126128
/24256254256Typical Class C

🌐 Special IP Ranges

IP or RangePurposeUsable by Host?Notes
127.0.0.1Loopback❌Test local machine
169.254.0.0/16Link-local (APIPA)❌Assigned if DHCP fails
224.0.0.0 – 239.255.255.255Multicast❌Used in routing, streaming, etc.
192.0.2.0/24Documentation/Test Net❌Reserved for training/examples
255.255.255.255Broadcast❌Sends to all devices on LAN

🧪 Quick Public vs Private Check

Starts WithType
10.Private
172.16–31Private
192.168.Private
ElsePublic (unless special)
Special IP addresses
TypeLook ForMeaning
Loopback127.x.x.xSelf-reference (testing your own machine)
Link-local169.254.x.xDHCP failed — fallback local address
Multicast224.0.0.0 – 239.255.255.255One-to-many traffic like streaming, routing updates
TEST-NET192.0.2.0/24, 198.51.100.0/24 etc.For training, books, documentation
Broadcast255.255.255.255Reaches every host on local network
If an IP:
  • starts with 127 → it’s a loopback
  • starts with 169.254 → link-local fallback
  • starts with 224–239 → multicast
  • looks like 255.255.255.255 → local broadcast
  • is 192.0.2.x → for examples only

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