断点续传是我们现在经常接触的概念，那么HTTP协议是如何支持断点续传的呢。我们先从一个例子来看看。
下面是一个断点续传的例子：（使用NetVampire得到）
I01-7-1219:19:23-------------------------Attempt1-------------------------
P01-7-1219:19:24Connectingto127.0.0.3...
P01-7-1219:19:24Connectedto127.0.0.3[127.0.0.3]
S01-7-1219:19:24GET/VS0515AI.EXEHTTP/1.1
S01-7-1219:19:24Connection:close
S01-7-1219:19:24Host:127.0.0.3
S01-7-1219:19:24Accept:*/*
S01-7-1219:19:24Pragma:no-cache
S01-7-1219:19:24Cache-Control:no-cache
S01-7-1219:19:24Referer:http://127.0.0.3/
S01-7-1219:19:24User-Agent:Mozilla/4.04[en](Win95;I;Nav)
S01-7-1219:19:24
R01-7-1219:19:24HTTP/1.1200OK
R01-7-1219:19:24Server:ZeroHttpServer/1.0
R01-7-1219:19:24Date:Thu,12Jul200111:19:24GMT
R01-7-1219:19:24Cache-Control:no-cache
R01-7-1219:19:24Last-Modified:Tue,30Jan200113:11:30GMT
R01-7-1219:19:24Content-Type:application/octet-stream
R01-7-1219:19:24Content-Length:15143086
R01-7-1219:19:24Connection:close
R01-7-1219:19:24
P01-7-1219:19:25Datatransferstarted
I01-7-1219:19:32JobStoppedbyuser
I01-7-1219:19:33Received5275648bytesin0:00:07(691435bytes/s)
I01-7-1219:19:40-------------------------Attempt2-------------------------
P01-7-1219:19:40Connectingto127.0.0.3...
P01-7-1219:19:40Connectedto127.0.0.3[127.0.0.3]
S01-7-1219:19:40GET/VS0515AI.EXEHTTP/1.1
S01-7-1219:19:40Connection:close
S01-7-1219:19:40Host:127.0.0.3
S01-7-1219:19:40Accept:*/*
S01-7-1219:19:40Pragma:no-cache
S01-7-1219:19:40Cache-Control:no-cache
S01-7-1219:19:40Referer:http://127.0.0.3/
S01-7-1219:19:40User-Agent:Mozilla/4.04[en](Win95;I;Nav)
S01-7-1219:19:40Range:bytes=5275648-
S01-7-1219:19:40
R01-7-1219:19:40HTTP/1.1206PartialContent
R01-7-1219:19:40Server:ZeroHttpServer/1.0
R01-7-1219:19:40Date:Thu,12Jul200111:19:40GMT
R01-7-1219:19:40Cache-Control:no-cache
R01-7-1219:19:40Last-Modified:Tue,30Jan200113:11:30GMT
R01-7-1219:19:40Content-Type:application/octet-stream
R01-7-1219:19:40Content-Range:bytes5275648-15143085/15143086
R01-7-1219:19:40Content-Length:9867438
R01-7-1219:19:40Connection:close
R01-7-1219:19:40
P01-7-1219:19:40Datatransferstarted
I01-7-1219:19:41JobStoppedbyuser
I01-7-1219:19:41Received1124756bytesin0:00:01(969617bytes/s)
第一次是普通的传输；第二次由于没有传完全，就发出了Range这个头部，从5275648字节开始传输（默认是按字节算），回应使用206状态值，表示现在开始部分传输，回复Content-Length头部，表示传输的部分，用字节记，然后就与普通传输没有区别了。
通过上面的例子，你应该了解HTTP断点续传的原理了吧。