Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!pasteur!ucbvax!HOGG.CC.UOREGON.EDU!jqj From: jqj@HOGG.CC.UOREGON.EDU Newsgroups: comp.protocols.tcp-ip Subject: default broadcast address Message-ID: <8807071832.AA07969@hogg.cc.uoregon.edu> Date: 7 Jul 88 18:32:26 GMT Sender: daemon@ucbvax.BERKELEY.EDU Organization: The Internet Lines: 47 A typical situation for many of us is a subnetted class B network running a variety of IP-based hosts, including many that are 4.3BSD and 4.2BSD based. In such an environment, one settable parameter on the 4.3BSD systems and others is the "broadcast address". There are many possible values for this; I am curious what other sites set it to. It is clearly desirable for all machines on a given subnet to agree on the broadcast address. 4.xBSD generates ICMP errors or erroneosly forwards Ethernet broadcasts with IP addresses it does not recognize as IP broadcasts; even the latest Tahoe 4.3BSD code recognizes as IP broadcasts only {net,0,0}, {net,subnet,0}, 255.255.255.255, 0.0.0.0, and the set broadcast address (and not both {net,-1,-1} and {net,subnet,-1} even though both are legal!). Earlier releases recognize only a subset of these. By getting everybody to agree you'll eliminate all those erroneous ICMP unreachables and ARP requests for your subnet broadcast address. You might even avoid a serious broadcast storm... What is not clear is what in an imperfect world to try to get everybody to agree to. Some possibilities: {net,0,0} the 4.2BSD standard, and still the SunOS 4.0 default (according to "man ifconfig"). Some people use this for backwards compatibility with 4.2, but it is a violation of the IP spec and 4.2 implementations are disappearing (right?). {net,subnet,-1} i.e. the subnet broadcast address. The 4.3 Tahoe default if you set a subnet mask but no broadcast address. Not too good if you plan to change your subnet mask or if there is confusion over what it is (e.g. if you're trying heterogenous sized subnets). {-1,-1,-1} i.e. the local broadcast address. Some IP implementations insist on generating this address, so it might also be a reasonable choice of standard. {net,-1,-1} i.e. the network broadcast address. Has the advantage of being immune to mis-set subnet mask problems. But not a good choice if you have routers that explode letter bombs and you want broadcasts to be subnet-local. In the past, I've standardized on {net,subnet,-1}. Charles Hedrick, the first of us to have analyzed the broadcast storm problem, has in the past recommended {net,0,0} as a concession to 4.2BSD [Chuck, am I taking your name in vain?]. So, my questions: (1) What have other sites standardized on? (2) What are the disadvantages, if any, of standardizing on the subnet broadcast address?