CCNA - Are private IP's routable?

Author

Are private IP's routable?

digitalsoul

2003-09-14, 8:39 pm

How do routers deal with the three private IP blocks, i.e,:

10.0.0.0 - 10.255.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255

My understanding that these IP's are not broadcasted across the Internet but are rahter presented using NAT address. But how do routers within an autonomus system, e. g, local rotuer deal with these IP blocks?

mikop

2003-09-14, 9:25 pm

same way.

they are not routable across the internet because they are explicitly excluded by isp, core etc, not because there is anything fundamentally unroutable with these range.

"Routers in networks not
using private address space, especially those of Internet service
providers, are expected to be configured to reject (filter out)
routing information about private networks. "

http://www.faqs.org/rfcs/rfc1918.html

for enterprise with internal routing using private addresses, they need to follow good guidelines, NSA

IP Address Spoof Protection
The filtering suggestions in this sub-section are applicable to border routers, and most
interior routers. With backbone routers, it is not always feasible to define ‘inbound’
and ‘outbound’.
Inbound Traffic
Do not allow any inbound IP packet that contains an IP address from the internal
network (e.g., 14.2.6.0), any local host address (127.0.0.0/8), the link-local DHCP
default network (169.254.0.0/16), the documentation/test network (192.0.2.0/24), or
any reserved private addresses (refer to rfc 1918) in the source field. Also, if your
network does not need multicast traffic, then block the IP multicast address range
(224.0.0.0/4). Apply this access list to the external interface of the router, as shown
in the transcript below.

http://www.nsa.gov/snac/cisco/guides/cis-2.pdf <--- good thing to read.

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