A network administrator configures the interface fa$0/0$ on the router R$1$ with the command ip address $172\mathrm{.}16\mathrm{.}1\mathrm{.}254255\mathrm{.}255\mathrm{.}255\mathrm{.}0.$ However, when the administrator issues the command show ip route $,$ the routing table does not show the directly connected network. What is the possible cause of the problem?