求助：Python本机网络通信程序，Client端和Server端不能通信？

Server端与Client端代码如下，当Client端用本地回路地址时127.0.0.1，虽然Server端与Client端都能运行，但不能通信。
我将host改为本机具体的IP地址时，就可以。
麻烦各位大侠，看看为什么？
小弟不甚感激！！！！

Server端代码：------
import socket

print 'Server'

host = ''
port = 50000
backlog = 5
size = 1024
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind((host,port))
s.listen(backlog)
while 1:
client, address = s.accept()
data = client.recv(size)
if data:
client.send(data)
print 'Server received:', data
client.close()

Client端代码：------
import socket

print 'Client'
#host = '192.168.0.101'
host = '127.0.0.1'
port = 50000
size = 1024
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((host,port))
s.send('Hello, world')
data = s.recv(size)
s.close()
print 'Received:', data

a = raw_input('Enter to continue...')