You can restrict access to your ssh server in many ways.
IMO the most important is to use ssh keys and disable password authentication.
See the following wiki pages for details
You can restrict access to a specific subnet in several ways. I will assume your ssh server is on subnet 192.168.0.0/16 with an ip address of 192.168.0.10 , adjust accordingly ;)
One line of defense is to use a router. Be sure to disable UPnP and do not allow port forwarding.
You can set several options in
/etc/ssh/sshd_config. One is the listen address. If You set a listen address on your subnet. A private ip address is not routable over the internet.
You can also use the AllowUsers
Somewhat related, you can also change the port
As outlined on the forums post, you can use TCP Wrapper . TCP wrapper uses 2 files,
/etc/hosts.allow and add your subnet
sshd : 192.168.0.
/etc/hosts.deny , and deny all
ALL : ALL
See also http://ubuntu-tutorials.com/2007/09/02/network-security-with-tcpwrappers-hostsallow-and-hostsdeny/
Last you can firewall your server. You can use iptables, ufw, or gufw.
sudo iptables -A INPUT -p tcp --dport 22 -s 192.168.0.0/16 -J ACCEPT
sudo iptables -A INPUT -p tcp --dport 22 -j REJECT
sudo ufw allow from 192.168.0.0/16 to any port 22
gufw has a graphical interface