A namespace is a way of scoping a particular set of identifiers.
Using a namespace, you can use the same identifier multiple times in
different namespaces. You can also restrict an identifier set visible to
particular processes.

For example, Linux provides namespaces for networking and processes,
among other things. If a process is running within a process namespace,
it can only see and communicate with other processes in the same
namespace. So, if a shell in a particular process namespace ran ps waux, it would only
show the other processes in the same namespace.

Linux network namespaces

In a network namespace, the scoped ‘identifiers’ are network devices;
so a given network device, such as eth0, exists in a
particular namespace. Linux starts up with a default network namespace,
so if your operating system does not do anything special, that is where
all the network devices will be located. But it is also possible to
create further non-default namespaces, and create new devices in those
namespaces, or to move an existing device from one namespace to
another.

Each network namespace also has its own routing table, and in fact
this is the main reason for namespaces to exist. A routing table is
keyed by destination IP address, so network namespaces are what you need
if you want the same destination IP address to mean different things at
different times – which is something that OpenStack Networking requires
for its feature of providing overlapping IP addresses in different
virtual networks.

Each network namespace also has its own set of iptables (for both
IPv4 and IPv6). So, you can apply different security to flows with the
same IP addressing in different namespaces, as well as different
routing.

Any given Linux process runs in a particular network namespace. By
default this is inherited from its parent process, but a process with
the right capabilities can switch itself into a different namespace; in
practice this is mostly done using the ip netns exec NETNS COMMAND... invocation, which
starts COMMAND running in the namespace named
NETNS. Suppose such a process sends out a message to IP
address A.B.C.D, the effect of the namespace is that A.B.C.D will be
looked up in that namespace’s routing table, and that will determine the
network device that the message is transmitted through.

Virtual routing and
forwarding (VRF)

Virtual routing and forwarding is an IP technology that allows
multiple instances of a routing table to coexist on the same router at
the same time. It is another name for the network namespace
functionality described above.