There are many ways to help protect your network from attack, one of the simpilist and most effective is actually to filter incoming and outgoing traffic from your network. An excellent place to start is to utilize the rfc's to define IPv4 addresses that are not or never will be in use on the public Internet and not allowing that traffic inbound. In the same vein, you can use the same information to limit what is allowed to leave your network, such as only IPv4 addresses that are legitimately routable on the public Internet.
This is not a new or unique solution but it is more commonly done at the service provider and larger enterprise level because those type of operations pay attention to the rfc's but also because they recieve much higher traffic loads on average traditionally. I believe that this technique is still useful for much smaller operations to use and is relatively simple to set up and maintain.
Here is a short list of rfc's to put in your firewall or edge router of addresses you should not be seeing from the Internet and ones that you should consider filtering out before sending traffic out to the Internet.
network RFC 1112
description - Host Extensions for IP Multicasting - in RFC 1700 also
network RFC 1700
description - assigned numbers - multicast, current, host, and reserved
network RFC 1797
description - Class A Subnet Experiment - may get reallocated - use the bogon list instead
network RFC 1918
description - reserved private IPv4 addresses
network RFC 2544
description - Benchmarking Methodology for Network Interconnect Devices
network RFC 3068
description - IPv4 reserved 6to4 IPv6 gateway services
network RFC 3171
description - IANA Guidelines for IPv4 Multicast Address Assignments
188.8.131.52 184.108.40.206 (covers 220.127.116.11 through 255.255.255.255)
network RFC 3927
description - Dynamic Configuration of IPv4 Link-Local Addresses - in 5735 above
network RFC 5735 (update of RFC 3330)
description - this rfc really collects all the other rfc with special use (reserved and limited IPv4 blocks) in a single doc, these is only a partial listing
18.104.22.168 255.0.0.0 (may be reallocated - use the bogon list instead just in case)
network RFC 5736
description - IPv4 Special Purpose Address Registry
network RFC 5737
description - reserved for test net
Here are some reference URL's to get you started to determine what you should apply for your needs.
In addition to using IP address list filters there are other protections you can take at the edge. You should consider putting more aggressive ICMP filters in and also filter specific IP protocol numbers from coming in or going out. I'll post more about that another time.