ip netns exec netns0 ip link set lo up
ip link add veth-default type veth peer name veth-netns0
ip link set veth-netns0 netns netns0

ip a | grep veth
6: veth-default@if5: mtu 1500 qdisc noop state DOWN group default qlen 1000
link/ether 56:78:d4:a5:8f:55 brd ff:ff:ff:ff:ff:ff link-netns netns0

ip addr add 10.0.3.1/24 dev veth-default
ip link set veth-default up

ip a | grep veth
6: veth-default@if5: mtu 1500 qdisc noqueue state LOWERLAYERDOWN group default qlen 1000
link/ether 56:78:d4:a5:8f:55 brd ff:ff:ff:ff:ff:ff link-netns netns0
inet 10.0.3.1/24 scope global veth-default
valid_lft forever preferred_lft forever

ip netns exec netns0 ip link set veth-netns0 up
ip netns exec netns0 ip addr add 10.0.3.2/24 dev veth-netns0

ping 10.0.3.2
PING 10.0.3.2 (10.0.3.2) 56(84) bytes of data.
64 bytes from 10.0.3.2: icmp_seq=1 ttl=64 time=0.102 ms
64 bytes from 10.0.3.2: icmp_seq=2 ttl=64 time=0.062 ms
64 bytes from 10.0.3.2: icmp_seq=3 ttl=64 time=0.062 ms
^C
--- 10.0.3.2 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2082ms

echo 1 > /proc/sys/net/ipv4/ip_forward
iptables -A FORWARD -o eno0 -i veth-default -j ACCEPT
iptables -A FORWARD -i eno0 -o veth-default -j ACCEPT
iptables -t nat -A POSTROUTING -s 10.0.3.2/24 -o eno0 -j MASQUERADE

ip netns exec netns0 route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
10.0.3.0 0.0.0.0 255.255.255.0 U 0 0 0 veth-netns0

ip netns exec netns0 ip route add default via 10.0.3.1
ip netns exec netns0 ping srv24x7.com
PING srv24x7.com (93.115.28.151) 56(84) bytes of data.
64 bytes from pagirnis.cloudlix.com (93.115.28.151): icmp_seq=1 ttl=58 time=1.19 ms
64 bytes from pagirnis.cloudlix.com (93.115.28.151): icmp_seq=2 ttl=58 time=1.47 ms
64 bytes from pagirnis.cloudlix.com (93.115.28.151): icmp_seq=3 ttl=58 time=1.65 ms
^C
--- srv24x7.com ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2002ms
rtt min/avg/max/mdev = 1.190/1.437/1.649/0.189 ms