August 24 at 8 a.m. we will change the network to strict enforcement of 
local address rules.  Til now, we have been running loose rules by providing 
a proxy ARP service to help misconfigured computers find campus network 
services.  We are making this change to assure that the network is not 
short circuiting one of the new enhancements in Windows XP SP 2. 

Proxy ARP is a service originally intended for operating systems that
do not factor subnet information when deciding which packets should be
sent to a router.  It has the side effect of helping systems that could
make this decision but for lack of proper information about the net mask.

One of the new XP firewall rules will block work group file sharing beyond 
the local subnet.  Hence it is important for the PC to accurately distinguish
between local and distant addresses.  The proxy arp service blurs that
distinction and would put misconfigured computers at additional risk
by exposing them to unexpected connections.

Properly configured computers will have the correct value for the subnet 
mask.  On Windows XP/2000 this can be viewed in a command window with the 
ipconfig command.  You'll get a display that looks like:

  Ethernet adapter Local Area Connection:

          Connection-specific DNS Suffix  . : ucsc.edu
          IP Address. . . . . . . . . . . . : 128.114.2.192
          Subnet Mask . . . . . . . . . . . : 255.255.255.0
          Default Gateway . . . . . . . . . : 128.114.2.252

If your computer's subnet mask is correct, the change will not affect you.
While proxy ARP interfers with SP 2 security, this change does not require
users to install SP 2.  After we make this change, computers with the wrong 
value will not be able to connect to some computers off their local subnet.

There is not a single right value for the netmask that covers the entire
campus.  Hence our recommendation: let DHCP take care of the details of
setting network parameters.  To do a manual check of the netmask, you can
verify the correct value for this important parameter in the tables at:

  http://toolbox.ucsc.edu/cgi-bin/dhcpstat/dhcpstatus_front_nolink.cgi

While the driver for this change is SP 2, computers of all flavors will
be affected.  If they have been hand-configured with the wrong mask
value, they will have net problems starting on the 24th.  Sun and SGI 
workstations historically have had more wrong mask problems than Windows
and Macs.