IP Addresses, Subnets, Routes and Gateways

What it is

An IP address identifies a device at Layer 3, and routes tell routers and hosts how to reach other networks.

The subnet mask or prefix length splits an address into a network part and a host part. A default gateway is the router that a host uses to leave its own network.

Every API call, SSH session and web request depends on these basics, so troubleshooting starts here.

How it works

A host compares the destination to its own network, and a router compares it to its routing table.

IPv4 addresses

An IPv4 address has 32 bits, written as four decimal numbers called octets, for example 192.168.10.5. Each octet runs from 0 to 255.

The subnet mask shows which bits are the network. A prefix length writes the same information as a count of bits.

The first address in a subnet is the network address, and the last is the broadcast address. The ones in between are usable for hosts.

For example, with 192.168.10.5/24:

• The network address is 192.168.10.0.
• The broadcast address is 192.168.10.255.
• The usable hosts run from 192.168.10.1 to 192.168.10.254.

Private addresses

Private IPv4 ranges are not routed on the internet. They are 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. NAT lets such hosts reach the internet through a public address.

Two special ranges are worth knowing. 127.0.0.0/8 is loopback, and 127.0.0.1 means this device. 169.254.0.0/16 is link local, and a host that cannot reach a DHCP server may give itself an address from it.

IPv6 addresses

An IPv6 address has 128 bits, written as eight groups of four hexadecimal digits, for example 2001:db8::1. Leading zeros can be dropped, and one run of zero groups can be replaced with a double colon.

IPv6 uses prefix lengths only, and a typical subnet is a /64. The address ::1 is loopback.

The default gateway

A host first checks whether the destination is in its own subnet.

• If it is, the host sends the frame directly to that device's MAC address.
• If not, the host sends the packet to its default gateway, and the router carries it onward.

If the gateway is missing or wrong, the host reaches local devices but nothing beyond them.

Routing tables

A router looks up the destination address in its routing table and picks the entry with the longest matching prefix. That is the most specific route.

R1# show ip route
S*    0.0.0.0/0 [1/0] via 192.0.2.2
C     192.168.10.0/24 is directly connected, GigabitEthernet0/0
C     192.168.20.0/24 is directly connected, GigabitEthernet0/1
C     192.0.2.0/24 is directly connected, GigabitEthernet0/2
S     10.50.0.0/16 [1/0] via 192.168.20.2

This IOS XE output shows connected routes marked C, static routes marked S, and a default route 0.0.0.0/0. A packet for 10.50.1.9 matches 10.50.0.0/16, and it is sent to 192.168.20.2. A packet for any other address uses the default route.

Routes come from three sources: directly connected networks, static routes that an administrator enters, and routing protocols such as OSPF and BGP.

R1(config)# ip route 10.50.0.0 255.255.0.0 192.168.20.2
R1(config)# ip route 0.0.0.0 0.0.0.0 192.0.2.2

The first command adds a static route to 10.50.0.0/16. The second sets a default route.

Checking a host

A Linux host can show its addresses and routes.

$ ip address show
$ ip route show
default via 192.168.10.1 dev eth0
192.168.10.0/24 dev eth0 proto kernel scope link src 192.168.10.5

The line starting with default is the default gateway. Windows has ipconfig and macOS has ifconfig for the addresses.

In automation

Scripts often need to work with subnets. Python's standard ipaddress module checks and calculates them.

import ipaddress

net = ipaddress.ip_network("192.168.10.0/24")
print(net.num_addresses)
print(ipaddress.ip_address("192.168.10.5") in net)

The first line prints 256, and the second prints True.