Routing over IP Alias'd Interfaces

[Michael Laing <mpl@ucook.com>]

My ISP is changing my server IP address. The old one will stay available
to me for awhile. I interface to the ISP via cable modem.

I thought a good way to transition seamlessly would be to use IP alias
to set up the new interface, then make the DNS changes and let them work
their way thru, then reconfigure to just use the new address.

I recompiled the kernel with IP aliasing, and used the appropriate
ifconfig commands as explained in the howto.

This is a busy machine with 2 ethernet cards. eth0 is used for the
internal network.

The current IP address is 206.5.73.128; the new one is 206.5.72.119.

Here's the output of netstat -nr (with a few changes to obscure the
actual addresses):

Destination     Gateway         Genmask         Flags   MSS Window  irtt
Iface
206.5.72.119    0.0.0.0         255.255.255.255 UH     1500 0          0
eth1:0
206.5.73.128    0.0.0.0         255.255.255.255 UH     1500 0          0
eth1
192.168.0.1     0.0.0.0         255.255.255.255 UH     1500 0          0
eth0
192.168.0.0     0.0.0.0         255.255.255.0   U      1500 0          0
eth0
206.5.73.0      0.0.0.0         255.255.255.0   U      1500 0          0
eth1
206.5.72.0      0.0.0.0         255.255.255.0   U      1500 0          0
eth1:0
127.0.0.0       0.0.0.0         255.0.0.0       U      3584 0          0
lo
0.0.0.0         208.3.73.1      0.0.0.0         UG     1500 0          0
eth1

My understanding is that both routers for both the 206.5.73.0 subnet and
the 206.5.72.0 subnet are 'there' on the other side of the cable modem.

When I ping the router for the 206.5.72.0 subnet (206.5.72.1) I get the
following:

PING 206.5.72.1 (206.5.72.1): 56 data bytes
ping: sendto: Operation not permitted
ping: wrote 206.5.72.1 64 chars, ret=-1

Why is this? What can I do to accomplish what I want?

Thanks for your help.

Michael Laing