Octet

Say this first

The flags in angle brackets are what the interface is configured and able to do: UP means administratively up, LOWER_UP means the driver sees carrier (layer 1 is up). state is the operational state the kernel computed: UP, DOWN, or UNKNOWN for things like loopback that do not report carrier. Then come the MTU, the queueing discipline, the MAC, and each address with its prefix, scope and lifetime.

Go deeper

A real example

Output

$ ip addr show dev ens192
2: ens192: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 00:50:56:a1:2b:3c brd ff:ff:ff:ff:ff:ff
    inet 10.0.10.21/24 brd 10.0.10.255 scope global dynamic ens192
       valid_lft 85912sec preferred_lft 85912sec
    inet6 fe80::250:56ff:fea1:2b3c/64 scope link
       valid_lft forever preferred_lft forever

2: is the interface index. Other commands refer to it (for example veth0@if7).
UP admin up, LOWER_UP carrier present. A cable pull shows NO-CARRIER and state DOWN while UP stays, because the admin state did not change.
mtu 1500, qdisc mq (multiqueue NIC), qlen 1000 is the transmit queue length.
inet 10.0.10.21/24 brd 10.0.10.255 scope global dynamic: address with prefix, broadcast, scope global (routable), and dynamic means it came from DHCP; valid_lft counts down the lease.
inet6 fe80::.../64 scope link is the IPv6 link-local address, built here from the MAC with EUI-64.

Scopes

global is usable anywhere, link only on this link (fe80::, 169.254/16), host only inside this machine (127.0.0.1 on lo).

The short form

Output

$ ip -br addr
lo               UNKNOWN        127.0.0.1/8 ::1/128
ens192           UP             10.0.10.21/24 fe80::250:56ff:fea1:2b3c/64

They will ask next

The interface shows UP in the flags but state DOWN. What is wrong?
It is admin up but has no carrier. Check the cable, the switch port (shut, err-disabled, wrong speed), the SFP, or the VM's virtual NIC connection. ethtool ens192 shows Link detected: no.
Why does lo show state UNKNOWN?
Loopback has no carrier concept, so the kernel cannot compute an operational state and reports UNKNOWN. That is normal, as it is for some tun and dummy devices.
How do you add a second address, and how does it show up?
ip addr add 10.0.10.22/24 dev ens192. Because it is in the same subnet as the first one, it is listed with scope global secondary. If the primary is deleted, secondaries in that subnet are deleted too unless net.ipv4.conf.all.promote_secondaries=1 is set.

Traps

Reading LOWER_UP as "the interface is enabled". It only means carrier is present.
Ignoring dynamic and valid_lft. A DHCP address with a short lifetime left can explain intermittent loss.

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