4000 Router Series Layer 3 - IP Services Configuration Guide

# Configure the IP address of interface Ethernet 1/1.

[Router] interface ethernet 1/1

[Router-Ethernet1/1] ip address 192.168.20.99 255.255.255.0

# Enable common proxy ARP on interface Ethernet 1/1.

[Router-Ethernet1/1] proxy-arp enable

[Router-Ethernet1/1] quit

After the configuration, Host A and Host D can ping each other.