ubuntu上虛擬機的網絡實驗

宿主機：ubuntu1204 desktop

虛擬機：ubuntu1204 server

虛擬化軟件： virtualbox 4.1.12

虛擬機網絡設置：

Adapter 1:

Intel PRO/1000 MT Desktop (Bridged adapter, eth0)

宿主機eth0沒插網線，利用ifconfig設置ip如下：

#ifconfig eth0 192.168.4.2/24

#ip a 

1: lo: mtu 16436 qdisc noqueue state UNKNOWN 

    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00

    inet 127.0.0.1/8 scope host lo

    inet6 ::1/128 scope host 

       valid_lft forever preferred_lft forever

2: eth0: mtu 1500 qdisc mq state DOWN qlen 1000

    link/ether 20:6a:8a:39:70:d3 brd ff:ff:ff:ff:ff:ff

    inet 192.168.4.3/24 brd 192.168.4.255 scope global eth0

3: wlan0: mtu 1500 qdisc mq state UP qlen 1000

    link/ether 20:7c:8f:61:82:27 brd ff:ff:ff:ff:ff:ff

    inet 192.168.1.103/24 brd 192.168.1.255 scope global wlan0

    inet6 fe80::227c:8fff:fe61:8227/64 scope link 

       valid_lft forever preferred_lft forever

虛擬機與宿主機的eth0橋接，虛擬機設置如下：

#ifconfig eth0 192.168.4.2/24

虛擬機ping宿主機

# ping 192.168.4.3

此處爲一直等待，沒有反映。

宿主機抓包顯示如下：

#tcpdump -i eth0 -n

15:19:25.452878 ARP, Request who-has 192.168.4.3 tell 192.168.4.2, length 28

15:19:26.451340 ARP, Request who-has 192.168.4.3 tell 192.168.4.2, length 28

15:19:27.451419 ARP, Request who-has 192.168.4.3 tell 192.168.4.2, length 28

15:19:28.463920 ARP, Request who-has 192.168.4.3 tell 192.168.4.2, length 28

15:19:29.463457 ARP, Request who-has 192.168.4.3 tell 192.168.4.2, length 28

15:19:30.463530 ARP, Request who-has 192.168.4.3 tell 192.168.4.2, length 28

15:19:31.463691 ARP, Request who-has 192.168.4.3 tell 192.168.4.2, length 28

15:19:32.463560 ARP, Request who-has 192.168.4.3 tell 192.168.4.2, length 28

15:19:33.463584 ARP, Request who-has 192.168.4.3 tell 192.168.4.2, length 28

宿主機ping虛擬機

#ping 192.168.4.2

此處爲一直等待，沒有反映。

虛擬機上抓包沒有任何信息：

#tcpdump -i eth0 -n

宿主機廣播

#ping 192.168.4.255 -b

此處爲一直等待，沒有反映。

虛擬機上抓包顯示如下：

tcpdump -i eth0 -n

問題：

爲何對宿主機和虛擬機都進行了正確的IP設置，卻不能相互ping通呢？