Create 2 network namespaces and bridge them and the default namespace together so that they can directly talk to each other via static IP addresses; also enable Internet access in all the namespaces via NAT.

network-namespace-demo.sh

#!/usr/bin/env bash

set -x

main_if=ens33
ping_count=1

echo 1 > /proc/sys/net/ipv4/ip_forward || exit 1

ip netns del ns1 > /dev/null 2>&1
ip netns del ns2 > /dev/null 2>&1
ip link del br0 > /dev/null 2>&1

ip netns add ns1 || exit 1
ip netns exec ns1 ip link set dev lo up || exit 1

ip netns add ns2 || exit 1
ip netns exec ns2 ip link set dev lo up || exit 1

# create the bridge
#ip0=10.0.1.254
ip link add br0 type bridge || exit 1
ip link set dev br0 up || exit 1
#ifconfig

# create a veth pair for the current host (default net namespace)

id=0

ip link del tap$id > /dev/null 2>&1

ip link add tap$id type veth peer name ns$id-veth || exit 1
ip link set ns$id-veth master br0 || exit 1

ip0=10.0.1.254
ip=$ip0
ip addr add $ip/24 dev tap$id || exit 1
ip link set dev tap$id up || exit 1
ip link set dev ns$id-veth up || exit 1

#ifconfig
#exit

# create the veth pair for ns1

id=1

ip link del tap$id > /dev/null 2>&1
ip link del tap${id}i > /dev/null 2>&1

ip link add tap$id type veth peer name ns$id-veth || exit 1
ip link set ns$id-veth master br0 || exit 1
ip link set tap$id name eth0 netns ns$id || exit 1

ip1=10.0.1.1
ip=$ip1
ip netns exec ns$id ip addr add $ip/24 dev eth0 || exit 1
ip netns exec ns$id ip link set dev eth0 up || exit 1
#ip link list
ip link set dev ns$id-veth up || exit 1

# enable internet access in the namespace via NAT

ip1i=10.$id.0.1
ip1i2=10.$id.0.2
ip link add tap${id}i type veth peer name ns${id}i-veth || exit 1
ip link set ns${id}i-veth name eth1 netns ns$id || exit 1

ip addr add $ip1i/24 dev tap${id}i || exit 1
ip link set dev tap${id}i up || exit 1

ip netns exec ns$id ip addr add $ip1i2/24 dev eth1 || exit 1
ip netns exec ns$id ip link set dev eth1 up || exit 1
ip netns exec ns$id ip route add default via $ip1i || exit 1

ip netns exec ns$id ifconfig
ip netns exec ns$id ip link list

iptables -P FORWARD DROP || exit 1
iptables -F FORWARD || exit 1
iptables -t nat -F || exit 1

iptables -t nat -A POSTROUTING -s $ip1i2/24 -o $main_if -j MASQUERADE || exit 1
iptables -A FORWARD -i $main_if -o tap${id}i -j ACCEPT || exit 1
iptables -A FORWARD -o $main_if -i tap${id}i -j ACCEPT || exit 1

ip netns exec ns$id ping -c $ping_count 8.8.4.4 || exit 1

#ip netns exec $ns ping -w 1 $ip0
#ping -w 1 $ip1

# create the veth pair for ns2

id=2

ip link del tap$id > /dev/null 2>&1
ip link del tap${id}i > /dev/null 2>&1

ip link add tap$id type veth peer name ns$id-veth || exit 1
ip link set ns$id-veth master br0 || exit 1
ip link set tap$id name eth0 netns ns$id || exit 1

ip2=10.0.1.2
ip=$ip2
ip netns exec ns$id ip addr add $ip/24 dev eth0 || exit 1
ip netns exec ns$id ip link set dev eth0 up || exit 1
ip link set dev ns$id-veth up || exit 1

echo "ping other nodes from inside ns$id..."
ip netns exec ns$id ping -c $ping_count $ip1 || exit 1
ip netns exec ns$id ping -c $ping_count $ip2 || exit 1
ip netns exec ns$id ping -c $ping_count $ip0 || exit 1

echo "ping other nodes from inside ns1..."
ip netns exec ns1 ping -c $ping_count $ip0 || exit 1
ip netns exec ns1 ping -c $ping_count $ip1 || exit 1
ip netns exec ns1 ping -c $ping_count $ip2 || exit 1

echo "ping other nodes from inside host..."
ping -c $ping_count $ip0 || exit 1
ping -c $ping_count $ip1 || exit 1
ping -c $ping_count $ip2 || exit 1
ping -c $ping_count 8.8.4.4 || exit 1

# enable internet access in the namespace via NAT

ip1i=10.$id.0.1
ip1i2=10.$id.0.2
ip link add tap${id}i type veth peer name ns${id}i-veth || exit 1
ip link set ns${id}i-veth name eth1 netns ns$id || exit 1

ip addr add $ip1i/24 dev tap${id}i || exit 1
ip link set dev tap${id}i up || exit 1

ip netns exec ns$id ip addr add $ip1i2/24 dev eth1 || exit 1
ip netns exec ns$id ip link set dev eth1 up || exit 1
ip netns exec ns$id ip route add default via $ip1i || exit 1

ip netns exec ns$id ifconfig
ip netns exec ns$id ip link list

iptables -P FORWARD DROP
iptables -F FORWARD
iptables -t nat -F

iptables -t nat -A POSTROUTING -s $ip1i2/24 -o $main_if -j MASQUERADE || exit 1
iptables -A FORWARD -i $main_if -o tap${id}i -j ACCEPT || exit 1
iptables -A FORWARD -o $main_if -i tap${id}i -j ACCEPT || exit 1

ip netns exec ns$id ping -c $ping_count 8.8.4.4 || exit 1

echo Success!