Computer Networking Cheatsheet

IP Addressing

Use this Computer Networking reference while you build software engineering projects, review code, or refresh the syntax you reach for most.

Anatomy of an IPv4 address

32 bits written as four dotted decimal octets, each 0-255. A mask splits it into a network part and a host part.

  192   .   168   .    10   .    37
11000000.10101000.00001010.00100101   address  192.168.10.37
11111111.11111111.11111111.00000000   mask     255.255.255.0  (/24)
11000000.10101000.00001010.00000000   network  192.168.10.0
11000000.10101000.00001010.11111111   broadcast 192.168.10.255

Rules that always hold in IPv4: the all-zeros host is the network address, the all-ones host is the broadcast address, and neither is assignable. A /24 therefore has 256 addresses but only 254 usable hosts.

Legacy classes

Classful addressing is obsolete (CIDR replaced it in 1993) but the vocabulary survives in exams, docs, and tool defaults.

Class	First octet	Default mask	Networks	Hosts per network
A	1-126	/8 `255.0.0.0`	126	16,777,214
B	128-191	/16 `255.255.0.0`	16,384	65,534
C	192-223	/24 `255.255.255.0`	2,097,152	254
D	224-239	none (multicast)	—	—
E	240-255	none (reserved)	—	—

127.0.0.0/8 is carved out of class A for loopback, which is why class A stops at 126.

Private ranges (RFC 1918) and other reserved blocks

Private addresses are not routable on the public internet, so they must be translated by NAT to reach it.

Block	CIDR	Addresses	Notes
10.0.0.0-10.255.255.255	`10.0.0.0/8`	16,777,216	RFC 1918, big flat enterprise space
172.16.0.0-172.31.255.255	`172.16.0.0/12`	1,048,576	RFC 1918, Docker default neighborhood
192.168.0.0-192.168.255.255	`192.168.0.0/16`	65,536	RFC 1918, home routers
127.0.0.0/8	loopback	16,777,216	`127.0.0.1` localhost
169.254.0.0/16	link-local	65,536	APIPA, DHCP failed · cloud metadata `169.254.169.254`
100.64.0.0/10	CGNAT	4,194,304	RFC 6598, carrier-grade NAT, Tailscale
0.0.0.0/8	this network	—	`0.0.0.0` means "any" when binding
224.0.0.0/4	multicast	—	groups, not hosts
192.0.2.0/24, 198.51.100.0/24, 203.0.113.0/24	documentation	768	RFC 5737, safe in examples

A common interview trap: 172.32.0.0 is public, because the private range stops at 172.31.255.255.

Static, DHCP, and link-local assignment

ip addr show                              # every address, all interfaces
ip -4 addr show dev eth0                  # just IPv4 on one NIC
ip addr add 192.168.10.37/24 dev eth0     # static, runtime only
ip addr del 192.168.10.37/24 dev eth0
ip route add default via 192.168.10.1     # gateway
dhclient -v eth0                          # request a lease

Persistent config depends on the distro: /etc/netplan/*.yaml (Ubuntu), nmcli con mod (NetworkManager, RHEL and Fedora), or /etc/network/interfaces (Debian classic). Runtime ip changes vanish on reboot.

Loopback and 0.0.0.0 when binding

Which address a server binds decides who can reach it, and this is the single most common "works locally, not from another machine" bug.

Bind address	Reachable from	Use when
`127.0.0.1`	only the same host	dev servers, DB behind a proxy
`0.0.0.0`	every IPv4 interface	containers, LAN services
`::`	every IPv6 interface, often IPv4 too via dual stack	modern servers
a specific IP	only that interface	multi-homed hosts, split traffic

ss -lntp                    # listening TCP sockets with the owning process
ss -lntp 'sport = :8080'    # who holds a port

Public IP, ASNs, and who owns a prefix

Public space is delegated by IANA to five RIRs (ARIN, RIPE NCC, APNIC, LACNIC, AFRINIC) which assign it to organizations, each identified by an autonomous system number.

curl -s https://ifconfig.me                  # your egress public IP
dig +short myip.opendns.com @resolver1.opendns.com
whois 8.8.8.8 | grep -iE 'orgname|netname|cidr'
whois -h whois.cymru.com " -v 8.8.8.8"       # ASN, prefix, country

Your egress IP is not your interface IP when NAT is in the path, and both differ from the address a load balancer or CDN shows the origin. That is what X-Forwarded-For exists to carry.