Post by b***@hotmail.comPost by Barry MargolinBGP is how ISPs learn the address blocks that belong to each other. ISP1
tells ISP2 "I have a route for 50.60.128.0/15". ISP2 merges that with
all the routing advertisements it has received from other ISPs, and uses
that when deciding where to send traffic. (This is an extreme
simplification, read the Wikipedia article on BGP for more details.)
1) Suppose I'm watching a Youtube video - it's true that different bytes of the same video could travel via different routes to get to me, right? Say, I'm in the UK, one byte could come straight from the US, and another via South America, say....? I mean, that's what TCP/IP does, right?
Yes. At least that's what IP does - packets are routed based on the
router's current view of the network. TCP reassembles those into the
order they were sent (and dealing with other things like lost
packets). In practice, there's a lot less variance in path than many
people seem to expect (although it certainly does exist).
Post by b***@hotmail.com2) What does the /15 mean, above? I've tried many times to understand what subnetting is, but I've never really got it. I think you have to understand internet routing before that, which, as I said, I don't *yet* (will soon! :) ), right....? I realise that that IS, probably, a separate question, I may open a new thread for that later, for the moment, could you just help me out...? :)
Look at it this way: every IP network is in a subnet, routing is
between subnets. The /15 in the above defines what addresses are in
the subnet. In this case the /15 means that the first 15 bits of the
32 bit IPv4 address define the subnet, so all the addresses from
50.60.0.0 to 50.61.255.255 are in that subnet. I suspect Barry meant
to use 50.60.128.0/17 as the example (which would define a subnet
containing all the address from 50.60.128.0 to 50.60.255.255, or
looking at the binary, addresses matching
00110010.00111100.1xxxxxxx.xxxxxxxx). In Barry's example, 50.60.128.0
is one address in that subnet.
Note that what's a single subnet at one level might be several smaller
subnets at another level. For example, a (very) big company might
have 50.60.128/17 assigned to it, which is the one subnet the world
sees, but internally that would be divided up into many smaller
subnets. An ISP with that assignment might, for example, dole out /23s
(blocks of 32 IPv4 addresses) to individual businesses using that ISP.
Again, the world would see the one large (/17) block), but once a
packet get into the ISP's network, the ISPs routers would figure out
to which individual company (/23) to route the packet.