Well, you have to have your own ASN to announce it, yes. :) And certainly, you have to get the other ISP to allow this announcement from you through their filters, but that's a normal part of establishing BGP routing with another provider.
But the point is that if Level3 announces 4.0.0.0/8, yes, you can announce 4.16.73.0/24 through them, and through Global Crossing as well. Level3 does have to forward your announcement to its peers de-aggregated, though; that is, if they just fold it into their 4.0.0.0/10 announcement, incoming traffic will prefer your /24 announcement through GX because the prefix is longer/more specific, and plus that gives Level3 no way to withdraw the announcement if the link goes down. Not everybody will happily let you punch holes in their nice, clean aggregate like that, potentially hosing their AS as a whole with flap dampening penalties and such if the link is flaky. However, it is normal order of business with Tier 1s.
But yes, if you have an ASN, you can announce a /24 or bigger directly. You can't do anything smaller, though I have the sneaking suspicion that may change. There are some cranky network operators out there who have not upgraded to equipment with the horsepower and RAM (most importantly RAM) to hold a full BGP view consisting of prefixes down to the /24 granularity, and will indeed filter higher, but they're techically misrouting -- their problem. In any case, that's not the norm, no.
If everybody filtered prefixes smaller than /19 or /20, then multihoming would be the province of a relatively small plutocracy of institutions. I haven't run the numbers any time remotely recently, but most announcements, by volume, are of prefixes smaller than /19 for sure.
If I'm reading "Prefix Length Distributions" right, 52% of all announcements are /24s, and average prefix length is 22.33.
Now, technically, what's being announced != what's being filtered by influential backbones. But it has to be pretty dang close, or ~52% of all announced networks would not be routable from Sprint. :-)
But the point is that if Level3 announces 4.0.0.0/8, yes, you can announce 4.16.73.0/24 through them, and through Global Crossing as well. Level3 does have to forward your announcement to its peers de-aggregated, though; that is, if they just fold it into their 4.0.0.0/10 announcement, incoming traffic will prefer your /24 announcement through GX because the prefix is longer/more specific, and plus that gives Level3 no way to withdraw the announcement if the link goes down. Not everybody will happily let you punch holes in their nice, clean aggregate like that, potentially hosing their AS as a whole with flap dampening penalties and such if the link is flaky. However, it is normal order of business with Tier 1s.
But yes, if you have an ASN, you can announce a /24 or bigger directly. You can't do anything smaller, though I have the sneaking suspicion that may change. There are some cranky network operators out there who have not upgraded to equipment with the horsepower and RAM (most importantly RAM) to hold a full BGP view consisting of prefixes down to the /24 granularity, and will indeed filter higher, but they're techically misrouting -- their problem. In any case, that's not the norm, no.
If everybody filtered prefixes smaller than /19 or /20, then multihoming would be the province of a relatively small plutocracy of institutions. I haven't run the numbers any time remotely recently, but most announcements, by volume, are of prefixes smaller than /19 for sure.
Take a look at this:
http://bgp.potaroo.net/as2.0/bgp-active.html
If I'm reading "Prefix Length Distributions" right, 52% of all announcements are /24s, and average prefix length is 22.33.
Now, technically, what's being announced != what's being filtered by influential backbones. But it has to be pretty dang close, or ~52% of all announced networks would not be routable from Sprint. :-)