IP Addressing & DNS

What is a Network?

A network is a group of computers and computing devices connected together through communication channels (cables, Wi-Fi, fiber). Networks allow devices to communicate, share resources, and exchange information. Most organizations have both an internal network and an Internet connection - the Internet itself is the largest network in the world, often called “the network of networks”.

IP Addresses

Every device attached to a network must have at least one unique IP (Internet Protocol) address - this is what enables routing packets to the right destination. IP addresses are assigned through RIRs (Regional Internet Registries) and come in two versions:

	IPv4	IPv6
Size	32 bits	128 bits
Example	192.168.1.10	2001:db8::1
Total addresses	~4.3 billion	~3.4×10³⁸
NAT needed?	Yes (address exhaustion)	No
Status	Still dominant	Growing adoption

Why didn’t we run out of IPv4? - NAT (Network Address Translation) extended IPv4’s life by letting many private addresses share one public IP. Your home router does this: your devices get private 192.168.x.x addresses that are invisible outside your network. The router holds the single public IP from your ISP.

IPv4 Address Types

• Unicast - delivered to a specific host (e.g., 140.211.169.4)
• Network - host portion set to all zeros; identifies the network itself (e.g., 192.168.1.0)
• Broadcast - host portion set to all ones; all members of the network listen (e.g., 192.168.1.255)
• Multicast - appropriately configured nodes subscribe to a group address (e.g., 224.0.0.2)

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Decoding IPv4 Addresses

A 32-bit IPv4 address is divided into four 8-bit sections called octets (each ranging 0–255). The address has two parts: a Network ID (Net ID) and a Host ID.

Historically, addresses were divided into classes based on the first few bits of the first octet:

Network Class	First Octet Range	Netmask	Notes
A	1–127	255.0.0.0	126 networks, ~16.7 million hosts each
B	128–191	255.255.0.0	16,384 networks, 65,534 hosts each
C	192–223	255.255.255.0	~2.1 million networks, 254 hosts each
D	224–239	-	Multicast
E	240–254	-	Reserved

Class A, B, and C in Detail

Class A (eno1 example: 1.0.0.0–127.255.255.255):

• First octet = Net ID; remaining three = Host ID
• First bit always 0 → 7 bits for network = 126 usable Class A networks

Class B (128.0.0.0–191.255.255.255):

• First two octets = Net ID; last two = Host ID
• First two bits always 10 → 14 bits for network = 16,384 networks

Class C (192.0.0.0–223.255.255.255):

• First three octets = Net ID; last octet = Host ID
• Most common for small networks (254 hosts max)

Subnet Masks

The netmask defines which bits belong to the network and which to the host. You can AND an IP with the netmask to extract the network address:

  172.16.2.17   IP address
& 255.255.0.0   netmask (Class B)
─────────────
  172.16.0.0    network address

Class	Decimal	Binary
A	255.0.0.0	11111111 00000000 00000000 00000000
B	255.255.0.0	11111111 11111111 00000000 00000000
C	255.255.255.0	11111111 11111111 11111111 00000000

Modern CIDR Notation

Modern networking uses CIDR (Classless Inter-Domain Routing) - the /prefix notation - which is more flexible than classful addressing:

192.168.1.10/24
└── Network: 192.168.1.0  (first 24 bits)
└── Host:    .10          (last 8 bits → 254 usable hosts)

CIDR	Subnet Mask	Usable Hosts
/8	255.0.0.0	~16.7 million
/16	255.255.0.0	~65,534
/24	255.255.255.0	254
/28	255.255.255.240	14
/30	255.255.255.252	2 (point-to-point links)

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Reserved / Private Address Ranges

Range	Purpose
10.0.0.0/8	Private (Class A space)
172.16.0.0/12	Private (Class B space)
192.168.0.0/16	Private (Class C space) - most common for home/office
127.0.0.0/8	Loopback - 127.0.0.1 is always “this machine”
0.0.0.0	Unspecified - used by DHCP before an address is assigned
255.255.255.255	Limited broadcast
169.254.0.0/16	Link-local (APIPA) - assigned when DHCP fails

Note

Private address ranges (10.x, 172.16–31.x, 192.168.x) are IANA-reserved and not routable over the internet. They require NAT to reach public networks.

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IPv6 Address Types

• Link-local (fe80::/10) - auto-configured on every interface; not routable outside the local link
• Global unicast (2000::/3) - publicly routable; the IPv6 equivalent of a public IPv4 address
• Multicast (ff00::/8) - one-to-many; replaces IPv4 broadcast
• Anycast - assigned to multiple interfaces; packet routed to the nearest one
• IPv4-Mapped - ::FFFF:a.b.c.d/96 - represents an IPv4 address in IPv6 space
• Loopback - ::1 (equivalent of 127.0.0.1)

IPv4 Address Allocation

• IP ranges are requested from ISPs or RIRs based on network size
• Static assignment: fixed, manually configured; required for servers
• Dynamic (DHCP): automatically assigned at boot; can change on reconnect

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ip Command Reference

ip is the modern replacement for the deprecated ifconfig, route, and arp.

# Interfaces
ip addr                          # show all interfaces and IPs
ip addr show eth0                # show specific interface
ip addr add 192.168.1.10/24 dev eth0   # add IP (temporary)
ip addr del 192.168.1.10/24 dev eth0   # remove IP

ip link show                     # show interface state
ip link set eth0 up              # bring interface up
ip link set eth0 down            # bring interface down

# Routing
ip route                         # show routing table
ip route show default            # show default gateway
ip route add default via 192.168.1.1 dev eth0    # set default route (temporary)
ip route add 10.0.0.0/8 via 192.168.1.254        # add static route (temporary)
ip route del 10.0.0.0/8                          # delete route

# ARP / Neighbours
ip neigh                         # show ARP table
ip neigh flush dev eth0          # clear ARP entries for interface

Tip

ip changes are temporary and lost on reboot. Use nmcli or NetworkManager connection profiles to make changes persistent.

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Name Resolution

Name resolution converts numerical IPs to human-readable hostnames (e.g., 104.95.85.15 → whitehouse.gov). Resolution happens in order, controlled by /etc/nsswitch.conf:

1. /etc/hosts - local static entries (checked first)
2. DNS servers in /etc/resolv.conf

/etc/hosts

127.0.0.1   localhost
192.168.1.10 db-server db01   # multiple aliases for same IP

DNS

If /etc/hosts can’t resolve a name, the system queries a DNS (Domain Name Server). DNS is a distributed system - any single DNS server only knows its zone of authority, but they cooperate to resolve any name globally.

The machine’s DNS is configured in /etc/resolv.conf:

search example.com aps.org   # appended to unqualified names
nameserver 192.168.1.1
nameserver 8.8.8.8

Note

On modern systems, /etc/resolv.conf is often auto-generated by NetworkManager or managed by systemd-resolved. Edit DNS settings via nmcli rather than editing the file directly.

DNS Query Commands

dig example.com              # full DNS lookup (most information)
dig example.com A            # explicit record type: A, AAAA, MX, TXT, CNAME, NS
dig -x 1.1.1.1               # reverse lookup (IP → hostname)
dig +short example.com       # just the answer - great for scripting
dig +trace example.com       # trace full resolution chain from root servers
dig @8.8.8.8 example.com     # query a specific DNS server

host example.com             # simpler output
nslookup example.com         # older; avoid with DNSSEC (can return wrong answers)

Example dig output annotated:

dig linuxfoundation.org

;; QUESTION SECTION:
;linuxfoundation.org.    IN    A              ← querying A (IPv4) record

;; ANSWER SECTION:
linuxfoundation.org. 524 IN    A  3.13.31.214 ← TTL is 524 seconds

;; SERVER: 10.139.1.1#53                       ← which resolver was used
;; Query time: 3 msec

Reverse lookup (IP → hostname) using +short:

dig -x 1.1.1.1 +short
one.one.one.one.

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Hostname Management

hostname                               # print current hostname
hostnamectl                            # full info: static, transient, pretty + OS details
sudo hostnamectl set-hostname myserver # set persistently (stored in /etc/hostname)
sudo hostnamectl set-hostname "My Server" --pretty

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Network Troubleshooting - Quick Checklist

Work outward from the local host:

Step	Command	What it tells you
1. Interface up?	ip addr show	IP assigned, state UP/DOWN
2. Network driver loaded?	lsmod | grep <driver>	Kernel module present
3. Reach gateway?	ping -c 3 $(ip route | awk '/default/{print $3}')	Local LAN reachable
4. Reach public IP?	ping -c 3 8.8.8.8	Routing works (bypasses DNS)
5. DNS working?	dig +short google.com	Resolver reachable
6. Trace the path	mtr 8.8.8.8	Per-hop loss and latency

Related

For network interfaces, routing, ping, traceroute, NTP, SSH, wget/curl, and FTP, see Network Tools & Services.